ON A 
NEW FORM OF LANTERN-GALVANOMETER 
/ 
By ALFRED M. 
Professor of Physics in the Stevens Institute of Technology, Hoboken, N. J. 
[From the American Journal of Science and Arts, Vol. Ill, June, 1872.] [From the American Journal of Science and Arts, Vol. HT, June, 1872.J 
ON A 
NEW FORM OF LANTERN-GALVANOMETER. 
By ALFRED M. MAYER, V-' 
Professor of Physics in the Stevens Institute of Technology, Hoboken, N. J. 
On the 21st of December, 1871, I delivered a lecture on 
Magnetism before the American Institute, at the Academy of 
Music, in the city of New York. It was necessary for the 
experimental discussion I then made of the earth’s magnetism 
to use a galvanometer, so constructed that the least deflection of 
its needle would be visible to a very large audience; at the 
same time the astatic condition of this needle had to be so 
controlled that it could readily be altered during the progress 
of the lecture ; while finally, the arrangement of the damping 
magnets had to be such as allowed me instantly to bring the 
needle into the magnetic meridian when disturbed therefrom, 
whenever I set in action the huge electro-magnet used on that 
occasion. Indeed, one of the principal uses to which this 
galvanometer was applied, in the lecture, was the exploration 
of the magnetic condition of the space surrounding this electro- 
magnet. This I accomplished by rotating, around the line of 
“the dip,” as an axis, wire coils at various distances and posi- 
tions, and leading the induced magneto-electric currents through 
the galvanometer. 
The lantern-galvanometer, which I will now proceed to de- 
scribe, I devised on the 13th of last November, and as subse- 2 
A. M. Mayer—New form of Lantern-Galvanometer. 
quent work with it has convinced me of its value in the lecture- 
room, I have decided to give it this formal publication. 
Referring to the figure; M is a plane mirror inclined 45° to 
the vertical. In front of this are the back condensing lenses of 
an oxyhydrogen lantern ; while the front lens of the condenser 
is placed in a horizontal position at c, above the mirror. The 
back condensing lenses are of such curvatures that when the 
calcium light is placed about two inches from the one nearest 
it, a nearly parallel beam issues from them to fall upon M, 
thence to be reflected to the upper condensing lens at c* on 
which rests a disc of glass on whose border is photographed a 
divided circle. In the center of this disc is a short needlepoint 
on which freely rotates a magnetic needle. Above the needle 
is the projecting lens L, the pencils from which are reflected in 
any desired direction by means of the plane mirror R, which 
revolves on a horizontal axis and has also a motion in azimuth 
around the axis of the lens.f 
The horizontal condensing lens is five inches in diameter, 
and the magnetic needle is four inches long. With this arrange- 
ment I have obtained sharp and bright images of the graduated 
circle 16 feet in diameter. 
To deflect this needle by means of an electric current I place 
as close to the condensing lens as possible the two vertical wire 
spirals S, S, formed of TV inch copper wire of square section, so 
as to bring the convolutions as close together as possible. The 
turns of the spirals are separated with very thin vulcanite rib- 
bon, coated with paraffine, and are wrapped on the faces of vul- 
* This arrangement of lenses, which is due to President Morton, gives a bright 
and uniformly illuminated field free from coloration. 
In the Quarterly Journal of Science, Oct. 1871, is a report of Prof. Morton’s ac- 
count of this invention [*' the vertical lantern ”], delivered before the American 
Institute, as follows: “ The original idea and general plan of the instrument shown, 
was, as the speaker stated, due to Professor J. P. Cooke, of Cambridge; his own 
work in connection with it being confined to the devising of a convenient me- 
chanical arrangement of parts, the improvement of the combination of condensing 
lenses with the reflecting lenses [mirror?] so as to secure a white and evenly 
illuminated field on the screen, and the discovery that an ordinary silvered mirror 
would serve for the final reflection as efficiently as a metal speculum or glass 
silvered by Foucault’s plan, which are so difficult to obtain and keep in order.” 
t In a college course of lectures it is sometimes convenient to reflect the image 
of circle and needle down on a white covered table below the class. The galva- 
nometer can then be placed on the lecture table. A. M. Mayer—New form of Lantern-Galvanometer. 
3 
canite discs. The spirals have an internal diameter of four 
inches and an external diameter of ten inches, and each contains 
49 feet of wire in 26 turns. The four terminals of the spirals 
are connecting screws, two of which serve to connect the spirals 
so that a current will circulate in the same direction in both. 
The spirals are so placed that a line joining their centers will 
pass through the center of the magnetic needle. 
The vertical-lantern rests on a base three feet and a half 
long, twith 'guides on its sides, between which slide* boards 
carrying two bar-magnets, A and B, 15 inches long and one 
inch in diameter, as shown in the figure. These magnets can- 
not only approach to and recede from the lantern, and thus 
alter their distances from the galvanometer-needle, but they can 
also rotate around their centers on vertical axes. The like poles 
of the magnets and of the needle point in the same direction; 
and by sliding the magnets to or from the lantern-needle we 
render the latter more or less astatic. Also, in case the needle 
should not hold to the meridian as you approach the magnets, 4 
A. M. Mayer—Neio form of Lantern-Galvanometer. 
it can be made to do so by rotating one or both of them in the 
horizontal plane; and thus, also, can be neutralized any exterior 
disturbance which may tend to deflect the needle from the 
magnetic meridian. 
The needle may be also rendered astatic, in the usual way, 
by suspending it by a silk fiber and attaching to this needle a 
wire which passes through a hole in the condenser and in the 
inclined mirror and carries beneath the latter another needle 
with poles reversed. 
In working with thermal currents we use a smaller needle 
and condenser which allows the spirals to approach nearer; but 
for thermal currents it is better to wind close around the needle 
a flat coil of only one wire in breadth, and to use a suspended 
astatic system of which the lower needle is the stronger and is 
under the control of the damping magnets.* The breadth of 
the coil used in this last device need not exceed th of an inch, 
and its image on the screen can answer for a rough zero point. 
I will now give a few experiments in which this galvanome- 
ter has been used ; and they will serve to show its usefulness. 
Experiment 1. A coil of feet in diameter, containing 40 
turns of 300 feet of T‘oth inch wire, was placed, with its plane, at 
right angles to “the dip.” Its terminals were connected with 
the galvanometer whose needle was rendered astatic by means 
of the the damping magnets. I now quickly rotated the coil 
180° around an axis at right angles to the direction of dipping- 
needle. The galvanometer needle was deflected about 12° by 
the magneto-electric current induced by the earth’s magnetism. 
Exp. 2. I placed the coil, used in Exp. 1, on a wooden wheel 
provided with a commutator and rotated it around an axis at 
right angles to the dip. The galvanometer-needle went steadily 
up to a deflection of 85° and was held there as long as the coil 
revolved. 
Exp. 3. The two cores of the large electro-magnet of the 
Stevens Institute of Technology were placed end to end, thus 
* The upper needle of this astatic combination swings in the interior of the coil 
which incloses both the needle and the condenser c; the lower needle swings un- 
der the inclined mirror M, and is attached to the upper needle by means of a stiff 
wire which passes through holes in the condenser and to the inclined mirror. In 
another combination I have placed this lower needle above the coil, and have 
“damped” it by means of a magnet placed above the reflector R. A. M. Mayer—New form, of Lantern-Galvanometer. 
5 
forming one iron bar, seven feet long and six inches in diam- 
eter. This was surrounded by its eight bobbins containing in 
all 2000 feet of j inch copper wire ; and through them was 
sent the electricity developed by the most advantageous com- 
bination of 60 plates of zinc and carbon, 10x8 inches. 
A coil of 20 inches in diameter formed of one turn of 2b inch 
wire was rotated 180° around a vertical axis 3.j feet from the 
end of the magnet The needle was deflected 3°. 
Exp. 4. A coil of 20 inches in diameter having 5 turns of 
inch wire was rotated 180° around a vertical axis, at a dis- 
tance of 3£ feet from end of magnet. Deflection of needle was 
30°. 
Exp. 5. Same as Exp. 4, only coil had 10 turns of wire in- 
stead of 5. Galvanometer needle deflected 50° to 60°. 
Exp. 6. A coil of 20 inches in diameter formed of 10 turns, 
2*0 inch wire was revolved 180° around a vertical axis feet 
from end of magnet. Deflection of 22°. 
Exp. 7. The coil used in Exp. 6, was placed three feet eight 
inches above center of axis of the magnet and revolved 180° 
around a vertical axis; the needle was deflected 80°. 
Exp. 8. A coil of feet in diameter, formed of 40 turns of 
300 feet of T\ inch wire, was placed 28 feet distant from the 
center of the magnet, and with its plane coinciding with the 
plane of the magnet’s equator. On rotating it around a verti- 
cal axis the needle was deflected 20°. 
The following experiments will show the excellent propor- 
tions (arrived at by a long series of experiments) of the coil 
used in Exps. 1, 2 and 8, for the evolution and study of the 
electric currents induced by the earth’s magnetism. 
Exp. 9. The coil used in Exps. 1, 2 and 8 was laid on a table 
and its terminals connected with a galvanometer, which is used 
in connection with Nobili’s thermo-electric pile. The needles 
of this instrument made one oscillation in nine seconds. I 
lifted the east side of the coil only six inches; the needle was 
deflected 10°. Lifting the same side nine inches, the needles 
went to 22°. I now placed the coil in a north and south verti- 
cal plane and suddenly tilting its top six inches to the east or 
to the west the needles went to 60°. Tilting the coil nine 
inches sent the needles with a blow against the stop at 90°. 6 
A. M. Mayer—New form of Lantern-Galvanometer. 
The advantages of the new galvanometer may be summed up 
in a few words. It gives on the screen a bright clear image of 
only the graduated circle and of the needle. It can readily be 
rendered more or less astatic to adapt it to the character of the 
electric currents worked with. The direction of its needle is 
completely under the control of the damping magnets; and, 
finally, it is of simple construction and can be rapidly adjusted 
to the requirements of any special experiment.