— s 
[From the American Jocrn. of Sciencf.s and Arts, Sept., 1809,] / If’j" 
ON THE NATURE AND DURATION 
OF THE 
DISCHARGE OF A LEYDEN JAR CONNECTED 
WITH AN INDUCTION COIL. 
By OGDEN N. 
Professor of Physics in Columbia College. 
[Read before the National Academy of Sciences, August, 1 
Part 1st. 
When the terminal wires of an induction coil of sufficient 
size are connected with the exterior and inner coating of a Ley- 
den jar, it is well known that the act of breaking the primary 
current charges the jar with electricity, and that a discharge 
will take place if the distance between the electrodes is not too 
great, otherwise the union of the two electricities is effected 
backward in the coil itself. The brightness of the light, and 
the ease with which it is generated, render it the most conven- 
ient means in our possession for the production of a nearly in- 
stantaneous illumination, while its richness in the violet and 
ultra-violet rays, make it suitable for experiments on fluores- 
cence and phosphorecence. Before, however, it can become 
useful for these and other delicate investigations, involving the 
apprecation of minute intervals of time, it is of course neces- 
sary that the nature and duration of the discharge itself should 
be studied, a matter which, so far as I know, has thus far not 
received attention. 
The case of a jar charged by an ordinary frictional machine 
is quite different from the above : here, during the act of 
charging and discharging, the two metallic coatings of the jar 
are entirely insulated, while when the induction coil is used in 
the manner above indicated, these coatings are during both acts 
in metallic connection, a circumstance which alone might be 
supposed to modify the duration of the discharge and its nature; 
besides this, it is known that the electricity from the coil con- 
tinues to flow for a considerable fraction of a second along the 
terminal wires into the jar, which fact renders it questionable 2 
whether the duration of the discharge of the jar itself may not 
he proportionately lengthened. Hence it is evident that it 
would not he safe to conclude without experiment, that the re- 
sults obtained by Feddersen with jars charged by ordinary fric- 
tional electricity, were applicable to those connected with an 
induction coil. 
Historical.—In the year 1835, Wheatstone published in the 
Philosophical Transactions, part II, page 583, an account of 
his celebrated experiment on the duration of the discharge of a 
Leyden jar charged by a common frictional machine. The light 
from the spark was received directly on a plane mirror revolv- 
ing at rates between 30 and 800 times in a second ; the mirror 
was driven by a set of multiplying wheels connected by strings. 
This apparatus was constructed by Mr. Saxton of Washington, 
who at that time was residing in London. The eye of the ob- 
server was placed near the mirror, and as the image of the spark 
was not sensibly drawn out by the rotation of the mirror, 
Wheatstone concluded that its duration was less than the one 
millionth of a second, a result which was accepted by the sci- 
entific world for about a quarter of a century, passing unques- 
tioned till th 3 publication in 1858 of the first of an admirable 
series of investigations of this subject by Feddersen ; (Pogg. 
Annalen, Bd. L03, seite 69.) This physicist employed a concave 
mirror with a radius of half a meter, driven by a train of 
toothed wheels, and obtained as high a rate of rotation as one 
hundred per second ; the spark was generated in one of the 
conjugate foci of the mirror, and its image, formed at the other 
conjugate focus on a plate of ground glass, was in a condition 
to be conveniently seen and measured by the naked eye or to be 
photographed. It was found that the image of the spark was 
drawn out by the revolving mirror into a whitish streak, from 
20 to 30 millimeters long, which when a large amount of elec- 
tricity was employed, was still farther lengthened by the addi- 
tion of a red tail of about the same dimensions, this latter 
being due to the gradual cooling of the heated particles. With 
ajar of 2’2 feet inner coating, the distance of the copper balls 
which served as electrodes being 1*5 millimeters, and all the 
connections as short as possible, the duration of the discharge 
exclusive of the red tail was ’00004 of a second. It was also 
found by him, that an increase of the size of the coating of the 
jar of the “ striking distance,” prolonged the duration of the 
discharge ; so, for example with a spark 375 millimeters long, 
the duration of the whitish portion was ’00007 of a second. 
A series of observations detailed at the end of this article, 
will point out a probable explanation of the great discrepancy 
existing between the results of Feddersen, which are undoubt- 3 
edly correct, and those of Wheatstone. Finally, Feddersen 
in the course of his investigation found abundant proof of 
the oscillatory nature of the electric discharge, which had 
been predicted from the results of a set of experiments by 
Prof. Joseph Henry as far back as 1842. 
Apparatus.—I arranged in the first place an apparatus like 
that of Feddersen, but it was soon found that io attain a rota- 
tion of one hundred per second, it was necessary in my case to 
employ a weight of 200 pounds, and in addition when using 
this form, I labored under the disadvantage of not being able 
to vary the size or curvature of the mirror readily ; in short it 
was found to lack flexibility, a quality more necessary in the 
present case than in the investigation which engaged the atten- 
tion of the German physicist. On this account I decided to 
employ a plane revolving mirror with a stationary achromatic 
lens. The addition of an extra wheel, with a few other me- 
chanical changes, converted the train of wheels furnished by 
Duboscq for Becquerel’s phosphoroscope, into an admirable 
means tor driving the revolving mirror, and with a weight of 
30 pounds, 300 rotations per second were readily obtained, the 
motion being smooth and sufficiently uniform. In order to 
measure the rate of rotation, the cylinder on the lowest Avheel 
was made to wind up a fillet of paper, upon which dots were 
made by an electro-magnetic apparatus regulated by a seconds 
pendulum, when a simple calculation furnished the rate of the 
wheel to which the mirror was attached. By this method it 
became possible to put to a sharp test the regularity of the rate 
of the train, which was found in all cases to be considerably 
greater than was at all necessary. These determinations were 
afterwards made with a watch having large seconds spaces, the 
dots being made by hand, as the refinement of a seconds pend- 
ulum and battery was found for my purpose to be superfluous. 
The mirrors employed were always plane, being sometimes 
silvered at the back, while for the most delicate observations 
the rear surface was painted with black varnish, so as to de- 
stroy the second reflection. They varied in size from 15 inches 
square, down to 2 by ’5 in., and were used sometimes double on 
the axis back to back, and at other times four mirrors were em- 
ployed together. The arrangement is seen in figure 1. The 
light diverging from S, was received by an achromatic lens L, 
of nine inches focal length ; it then fell on the plane mirror at 
M, and was finally brought to a focus on the plain or ground 
glass at G, which was distant from the mirror about 16 inches. 
For the purpose of measuring the extent to which the image 
of the spark had been drawn out by the motion of the mirror, 
a scale consisting of bright lines on a dark ground, was pro- 4 
jected on the ground glass, as seen in the figure at CC, the 
flame of a small lamp furnishing the illumination. The spark 
was generated between the balls or wires of a spark-microme- 
ter, graduated in millimeters ; this was placed of course in one 
of the conjugate foci of the achromatic lens. An automatic 
arrangement for breaking the circuit from three to six times 
in a second, was employed, mercury and alcohol being used 
in it. The coil was a large one by Ritchie of Boston ; it was 
excited during these experiments by two cups of the Bunsen 
form ; the simple sparks furnished by it did not at the time 
exceed seven inches in length. The Leyden jar had a coat- 
ing of 114 4 square inches, and all the connections were made 
as short as possible, so as to offer the least amount of resis- 
tance to the flow of the electricity. The experiments were 
usually made in a room not entirety darkened, as in a perfectly 
dark room it was found difficult to preserve the proper “ accom- 
modation ” of the eyes. 
If no means are employed to regulate the moment at which 
the discharge takes place, it is evident that images of the spark 
will be thrown on the ground glass only when the mirror hap- 
pens to be in a favorable position, still as several of them oc- 
curred in a second, and as the mirror was always double it did 
not very often happen the weight ran entirely down before a 
single observation had been made. I selected a certain portion 
of the ground glass along the line of images, and exclusively 
fixed the attention on it, and disregarding the images formed 
in its neighborhood waited for their appearance in this locality. 
If the precaution be neglected, much time will be wasted in 5 
accumulating observations ot comparatively little value. With 
velocities up to 223 per second, I succeeded in causing the dis- 
charge to take place when the mirror was in the proper position, 
by attaching to its axis a brass wire W, which was made to 
complete the connection between the exterior and inner coatings 
of the jar. The arrangement is seen in figure 1, and it was 
found quite valuable in facilitating the use of the eye-pieces, 
which were employed to magnify the lengthened images of the 
spark, or to examine the micrometer described at the end of 
this article. I had been apprehensive that this plan would not 
answer, as it evidently assumes that the jar remains charged 
a fraction of a second before it discharges itself backward 
through the coil. 
Appearance presented by the spark, viewed with the stationary 
mirror. 
(a) With brass halls as electrodes. Here the spark presented 
its ordinary well known appearance, being merely magnified by 
the eye-piece. 
Q>) With pointed wires as electrodes. When the “ striking 
distance” was one millimeter, 
the form was that represented 
in figure 2 at A, viz : a cen- 
tral white portion surrounded 
by a yellowish envelop, which 
in the figure is shown by the 
coarse shading ; the second en- 
velop more densely shaded, 
being green and much inferior 
in brightness. With a “ striking distance” of ten millimeters, 
the green envelop was produced only in the neighborhood of 
the electordes. (see B, fig. 2.) In both cases a number of 
brilliant minute jets of white light were seen to issue from the 
electrodes ; these are only partially shown in the figures. The 
green color was due to the heated particles of metal, and the 
bright points of light to successive discharges, as will be shown 
farther on. 
(c) With fine platinum wires. The appearance was the same 
as the last, except that the green tint was replaced by a gray 
or violet-gray hue. These observations were made with a mir- 
ror of plane glass, the rear side being blackened, which reduced 
the intensity of the light adequately for the purposes of dis- 
tinct vision. The ground glass was replaced by a polished 
plate, which supported an eye-piece magnifying five diameters. 6 
(a) With brass balls as electrodes. With a “ striking dis- 
tance” of one or two millimeters, the image being thrown on 
the ground glass and viewed by the naked eye, it was found 
that the spark was drawn out into a streak one and a half or 
two inches long, according to the rate of the mirror ; 
the portion at W, (fig. 3) was white ; this shaded 
into a brownish yellow tint, (B Y) the latter passing 
into a pretty distint green, (G.) It will be lound by 
comparison that this result is exactly that obtained by 
Feddersen with frictional electricity, except that in 
my case the green color of the tail seem to have been 
much stronger, which is explained by the lower inten- 
sity of the illumination. 
With the unaided eye and ground glass, nothing additional 
of a remarkable character could be discerned, but on using the 
polised plate of glass and an eye-piece magnifying five diam- 
eters, a series of bright dots on each side of the streak became 
visible in the positions indicated by the dots in figure 3. These 
resemble the photographs obtained by Feddersen with fric- 
tional electricity. Here it is seen that the bright points before 
spoken of, correspond to a series of discharges following the 
first. 
(/;) With platinum points. Using the ground glass, I found 
that the form of the streak was some times identical with that 
produced from brass balls, the green 
color merely being replaced by a vio- 
let-grayish tint. The form, however, 
was more often quite different, as 
shown in figure 4, the change in 
shape being due to the partial sup- 
pression of the tail, and to its irreg- 
ular production ; with low velocities 
the shape was often as in the small- 
est of the three sketches. 
With eye-piece, polished glass 
and high velocities, the series of 
bright points was seen beautifully 
developed. 
(c) With brass points, this same figure was produced, yet not 
quite so distinctly. The tail was of course green. 
Appearance presented by the rev ole in g mirror. 7 
Total duration of the discharge. 
(a) Using brass balls. 
Rate of rotation 
per second. 
Number ot 
observations. 
Distance of elect- 
rodes in millimeters. 
Duration in 
seconds. 
109* 
6 
I 
•000050 
291- 
5 
2 
*000028 
294* 
9 
1 
•000017 
120* 
0 
1 
•oooooo 
90- 
8 
1 
•000023 
no- 
15 
1 
•000022 
No red tail was ever observed in my experiments, the amount 
of electricity employed not being sufficient for its production. 
The difference in these observations arises partly from the faint- 
ness of the terminal portion of the tail, and also from the fact 
that its length is a little variable even in discharges following 
each other in quick succession. Still on the whole, these re- 
sults correspond pretty well with those obtained by Feddersen, 
when it is remembered that the coating of the jar employed by 
me, was less than one half the size of that used by him, which, 
as well as shortening the “ striking distance/' tends to lessen 
the duration of the discharge as proved by his observations. 
Hate of rota- 
tion per second. 
No. of observations. 
Distance of elect- 
rodes In millimeters. 
Duration In 
seconds. 
100 
10 
1 
*000025 
48 
10 
1 
•000028 
(b) Using brass points. 
(c) When platinum points were used as electrodes, with a 
striking distance of three millimeters, the duration, as obtained, 
varied between ’00004 and ’00014 of a second : the variation 
seems to be owing to the faintness and narrowness of the tail, 
which rendered observation difficult and rather uncertain. 
As it had all along been evident that the main illuminating 
power of the spark was concentrated in the first explosive act, 
I made still further efforts to determine the duration of the 
white and yellow portion of the discharge. The image of the 
spark, using platinum wires as electrodes with a striking dis- 
tance of five millimeters, was thrown on the ground glass, a 
set of observations being made with the naked eye, and also 
with the aid of the magnifying lens. The color was found to 
be rather brownish-yellow, shading off* and becoming fainter in 
illumination toward its farther boundary. This yellow flame- 
like portion was not nearly so bright as the white band, the 
latter being to all appearance unaffected by the motion of the 
mirror. The appearance was that indicated in figure 4, the 
Duration of the white and yellow portion. 8 
farther boundary resembling the irregular outline and color of 
a burning splinter of wood. 
The duration was found to vary from 
•000003 
•000007 
of a second. The rate of rotation was 206 per second. De- 
terminations of this same point had previously been made at 
an early stage of the work ; their result was 
•000008 
•000006 
of a second. There can be no doubt but that the duration of 
the yellow portion in individual sparks varies within limits like 
the above as can be seen by a comparison of successive dis- 
charges. 
Duration of the ichite band.—Up to this time I had been 
able to obtain no evidence that the duration of the white band 
was other than instantaneous, indeed with this mode of experi- 
menting the contrary cannot be proved. The preceding analy- 
sis had shown that a large portion of the light of the spark re- 
sided in this while band, i. e., in the first discharge, and when 
points were used as electrodes its superior brightness to some 
extent isolated it from the rest of the streak To ascertain 
whether it was to be regarded as a separate act by itself, or as 
only the beginning of a continuous action lasting *000005 of a 
second or longer, I made the following simple experiment : in- 
stead of receiving the light of the spark directly on the achro- 
matic lens L, it was used to illuminate a strip of white paper 
•14 of an inch in breadth ; this was placed horizontally, the 
direct light from the spark being prevented from reaching the 
lens by a screen. The tl striking distance” was 1*3 millimeters. 
With low rates of rotation, the appearance of the image of the 
band viewd by an eye-piece magnifying five diameters, was 
quite unaltered ; as the rates of rotation were increased the il- 
lumination of the image of the band grew fainter, but with 
the highest velocity, 223 per second, it was distinctly visible 
and of the same breadth, as far as could be ascertained by the 
use of the micrometer. This proves of course that the first 
discharge is a simple act, and isolated from those that follow. 
Next, as an appropriate termination to this experiment, I 
blackened all the surface of the strip of white paper except so 
much as served to form a square. On examining the image of 
this with the same rate of rotation, the square was tolerably 
visible : it was followed by a faint flame-shaped tail, corres- 
ponding to the yellowesh portion of the discharge. 
Having thus proved the existence of an isolated discharge, 
at once the first and most brilliant act in the (xplosion, a 9 
method was contrived for measuring its duration, or at all 
events for setting a limit to it on at least one side. Ordinary 
micrometric methods, conducted on the naked spark or on a 
strip of paper illuminated by it, were out of the question, and 
would have served no purpose except to lead the observer into 
error, so I contrived the following plan, which is moderately 
easy of application and safe. 
A small piece of cardboard was ruled with two black lines ; 
their distance apart was •0067 inches, and they were separated 
by a space exactly equal to the width of a single line. A small 
dividing engine was used for their production ; they were tested 
with a microscope. These lines were illuminated by the spark, 
and their image examined by an eye-piece magnifying five di- 
ameters. The breadth of the image of a single line on the 
ground glass was ’Oil inches, that is, each line subtended an 
angle of 2' 24", leckouing from the mirror. With a velocity 
of 223 per second the mirror is able to move the reflected image 
through 2' 24" in the -00000024 of a second. If now this first 
discharge had actually lasted this minute portion of time, it is 
evident that the motion of the mirror would just have carried 
the image of one of the black lines forward, so that at the end 
of this infinitesimal period, it would have occupied the space 
where just before the white line had been traced : hence owing 
to the retention of impressions by the retina the white central 
line would have been obliterated, and in place of three lines, 
a gray band would have been seen. On the other hand if the 
duration had been only T’7 or of the above mentioned inter- 
val, the white line not having been much encroached on, would 
still have remained visible. The correctness of the above rea- 
soning can be experimentally proved, by means of a revolving 
disc of card board with a single slit cut in it, lines being drawn 
on its white side, and viewed by reflection with a mirror through 
the slit, the blackened side of the disc being turned towards 
the eye. 
To facilitate matters, three sets of these lines were drawn on 
the small cardboard at considerable distances apart to prevent 
confusion, and while illuminated by the electric spark were ex- 
amined with increasing velocities. With low rates they were 
quite unchanged in appearance, with a velocity as high as 102 
per second, a duration of the first discharge of ’00000052 sec- 
onds would just have obliterated them ; they were however 
perfectly distinct, though more faintly traced : the rate of ro- 
tation was then by degrees carried up to 223 per second, when 
the lines were still distinctly visible, though of course with less 
contrast between the white and black, than was the case with 
low velocities or a stationary mirror. 10 
This experiment proves then, that the duration of' the light 
accompanying the first discharge is considerably less than 
•00000024 of a second, probably less than half this period, or 
less than one ten millionth ot a second of time. To make 
the observations required some patience, as it was necessary 
to use an eye-piece, the ground glass being replaced by a 
plain polished plate, and it was seldom that the image of 
the lines fell exactly in that portion of the field to which 
the attention was directed. To obviate this difficulty, I af- 
terward covered a piece of cardboard half an inch broad with 
alternate white and black lines of this character, their real di- 
ameter was *0075 in., that on the plate of glass being 0125 in. 
With these I repeated the above mentioned set of experiments 
with rapidity, obtaining with ease the same result, as some of 
the lines were almost certain to he in the right part of the field 
of view. It is thus shown, that the first act of the electric ex- 
plosion lasted through an interval of time so short as to he 
immeasurable with the means then at my disposal ; it is not 
impossible that it may still be reached by the use of finer lines, 
and a lens of greater focal length. 
From the foregoing then it appears that if a jar having a 
metallic coating of about 100 square inches, he connected as 
above described with an induction coil, its discharge will be 
effected by a considerable number of acts, of which the first is 
by far the most intense. Farther, the metallic particles heated 
up by the first discharge to a white heat, almost instantly as- 
sume a lower temperature, marked by a corresponding change 
from white to brownish yellow, and as their temperature con- 
tinues to fall the tint changes in the case of brass electrodes 
to green, in that of platinum to a gray or violet-gray. A spec- 
troscopic examination of these isolated tints would be interest- 
ing but not without difficulty. These observations farther 
demonstrated the fact, that two ten millionths of a second is 
an interval of time quite sufficient for the production of dis- 
tinct vision. 
« When the light from the spark is received directly on a 
phme revolving mirror, and viewed by the eye as in Wheat- 
stone’s original experiment, only the white unanalyzed portion 
of the spark is ordinarily visible ; at least in repeating the ex- 
periment a few times, it is all that I saw. Its form is of course 
that of the spark itself. In all probability this is also the case 
in a jar charged by frictional electricity, and may serve to ex- 
plain the great discordance between the results obtained by 
Wheatstone and Feddersen, the method used by the former 
furnishing only a view of the first act, the eye being too much 
dazzled to perceive those that are subsequent and of far less 
intensity.