24 A. G. Bel—An Induction Balance 
tone when the metallic disk passed the points marked 1, 2 and 
8 in the illustration, but the loudest effect was produced when 
the coin crossed the area marked “2,” where the two coils 
overlapped. 
After my return to America I embodied these and other 
results in a paper ‘ Upon New Methods of Exploring the 
Field of Induction of Flat Spirals,” which was read before 
this Association at the Saratoga Raabe in August, 1879. 
Practical application.— While brooding over the problem of 
the detection of the bullet in the bod of President Garfield, 
these experiments made in England returned vividly to ay 
mind. It seemed to me that if the overlapping area “2” of t 
two coils shown in fig. 1 could be brought over the seat of in 
bullet without disturbing the relative positions of the coils, the 
telephone would probably annouuce the presence of the bullet 
by an audible sound 
A crude experiment was at once made to test the idea. A 
large, single-pole electro-magnet (the core of which was com- 
of fine iron wires) was used in place of coil 
A (fig. 1); and a small coil of fine wire taken from a han 
rons. was arranged a little to one side of the pole to rep- 
resent eat . The small coil being connected witb a tele- 
one, a battery current was passed through the coil of the 
electro-magnet, and the battery circuit was made and broken 
by an assistant. 
nder these circumstances a much better balance was ob- 
tained than could possibly have been anticipated. Upon now 
bringing a leaden bullet near the small coil, a distinct ticking 
sound could be heard from the telephone each time the battery 
circuit was made and broken 
Being absent from my laboratory, and without facilities for 
proper experiment, I communicated my ideas to Mr. Charles 
Williams, Jr., of Boston, manufacturer of electrical and tele- 
phonic apparatus, who kindly placed ni resources of his large 
establishment at my service; and, at great personal incon- 
venience, delegated his best yorkie to attend to my experi- 
ments. 
Upon attempting to devise an appropriate form of apparatus 
for the special purpose in view I saw that there were great 
practical difficulties in the way of utilizing the arrangement 
shown in fig. 1, and it occurred to me that the apparatus of 
Prof. Hughes might perhaps be employed with more advantage 
as the basis of my experiments. In the a form of 
Hughes’ induction balance four coils are used, as shown in fig. 
2. Through the agency of a Hughes micro One the ticking 
of a clock is made to create an electrical disturbance in the 
voltaic circuit containing the two primary coils (A C) and a 
