Sor detecting Metallic Masses in the Human Body. 28 
Another and superior arrangement for the same purpose is 
the well-known induction balance of Prof. D. E. Hughes.* 
The Static Induction Balance of J. E. H. Gordon + though 
primarily intended for experiments upon specific inductive 
capacity, might also, perhaps, be employed in the same class of 
vestigations. 
My own attention was directed to the balancing of induction 
a number of years ago by the disturbing noises produced in 
the telephone by the operation of telegraphic instruments upon 
lines running near the telephone conductor. 
The difficulty was remedied by using two conductors,instead 
of one, and by so arranging them with reference to the dis- 
turbing wires that the currents induced in one of the telephone 
conductors were exactly equal and opposite to those induced 
in the other. In this way an induction balance was produced 
and a quiet cireuit secured for telephonic purposes. This 
method was patented in England in November, 1877, and dur- 
ing the whole winter of 1877-8 I was engaged in London upon 
experiments relating to the subject. 
in the course of these researches I made frequent:use of flat 
ee of insulated wire, like those employed by the late Prof. 
enry} in his experiments upon induction. : 
My method was to pass a rapidly interrupted voltaic current 
through one flat spiral while I examined its field of induction 
by means of another flat spiral connected with a telephone. 
The currents induced in the latter coil produced a musical tone 
from the telephone. 
: Fig.2. 
Positions making a right angle with one another. 
_ It was also noticed that when a position of silence was estab- 
lished a piece of metal brought within the field of induction 
Caused the telephone to sound. This effect was most marke 
When the two flat spirals were in close proximity, and were 
arranged with their planes parallel, as shown in fig. 1 
hen a silver coin, such asa half-crown or florin was passed 
across the face of the two coils, the silence of the telephone 
was broken three times. The instrament emitted a musical 
* Phil. Mag., July, 1879, vol. ii, p. 50. } Phil. Trans, for 1879, p. 417. 
¢ This Journal, xxviii, 329; xxxviij, 209; xli, 117. 
