OF AETS AND SCIENCES. 
XXIII. 
A NEW FORM OF INDUCTIVE APPARATUS. 
Henry P. Bowditch. 
Presented, Oct. 12, 1875. 
The inductive apparatus commonly used in physiological laboratories 
is the so-called “ Sledge Apparatus ”of Du-Bois Reymond. In this 
instrument, the intensity of the induced current is regulated by varying 
the distance between the two coils, their axes being always kept in the 
same straight line. By this method, very feeble currents can only be 
obtained by separating the coils to a considerable distance, and an in- 
strument made to permit this separation has often an inconvenient 
length. 
In the instrument here presented, and which is figured in the 
accompanying wood cut, this difficulty is obviated by allowing the 
secondary coil, as soon as it has been withdrawn enough to he fairly 
free of the primary coil, to rotate round a vertical axis. In this way, 
the intensity of the induced current may he reduced to any desired 
degree, zero being obtained when the coils are at right angles to each 
other. The effect of simple rotation of the secondary coil, regarded 
by itself, would doubtless he to cause the intensity of the induced 
current from that coil to vary in the same proportion with the cosine 282 
PROCEEDINGS OF THE AMERICAN ACADEMY 
of the angle of rotation. This effect is, however, complicated by the 
variation in the distance of the different parts of the two coils from 
each other which attends this rotation. It is accordingly found that 
the curve which represents the actual variations in intensity (obtained, 
according to Fisk’s method, by measuring with the galvanometer the 
intensity of single induction shocks at every ten degrees of revolution 
of the secondary coil *) differs from the curve of cosines in being 
slightly convex towards the abscissa just before it reaches that line. 
The scale with which the instrument is provided indicates the in- 
tensity of the induced current at different positions of the secondary 
coil, expressed in terms of an arbitrary unit employed in the gradua- 
tion of a large “ sledge apparatus,” in use at the physiological labora- 
tory, and similar to the unit adopted in German laboratories for the 
graduation of similar instruments. 
* See Cyon’s Methodik der physiologischen Experimente, p. 879.