THE AUDIOMETER. 208 
for hearing, and is divided into twenty centimetres, each of which is 
subdivided into ten, thus giving 200 units. The secondary coil depends 
entirety upon the current which is induced in it by the passage of a 
current through the primary coil, and is not in connection with the 
current from the battery, The telephone is in the circuit of the induced 
eurrent, and has no other source of electricity. The microphone is nothing 
more than a small apparatus by means of which contact may be made or 
broken. For instance, a small electric bell, deprived of its bell, becomes 
a microphone in the sense in which it is applied to the audiometer. 
The law from a knowledge of which this instrument was evolved is 
that a current passing through a wire will cause, within a certain 
distance, a current of an opposite character and direction in another wire 
placed near it. Ifa telephone is placed in the primary circuit, having a 
contact breaker or microphone in the same circuit, then every make or 
break sound will be heard in the telephone. Every time that contact is 
made or broken the secondary coil receives or loses an induced current, 
consequently the sounds produced by making and breaking contact are 
heard in the telephone, which is in the circuit made by the induced 
current. If, then, the secondary coil be steadily and slowly moved away 
from the large primary coil, the induced current becomes weaker and 
weaker, the sounds produced by the microphone or contact breaker become 
fainter and fainter, until the secondary coil arrives at a point called zero 
on the scale, where no sound can be heard at all. This is the place where 
the two primary coils exactly balance the secondary coil, which lies 
between them, andis also the average limit of hearing power in the 
healthy adult. Now, if the secondary coil be moved still further towards 
the lesser primary coil, the induced current begins to return, but is 
weaker than that which is induced by the larger primary coil. 
By this instrument many morbid conditions, a knowledge of which 
is indispensable to a correct diagnosis, can be observed with an amount 
of cértainty comparable to that derived from the use of a reliable 
thermometer in fevers. Throat deafness may now be diagnosed from 
deafness resulting from disease of the external ear, and the actual 
impairment ascertained. A chart may be keptof the daily progress of 
the case, and from it, in the course of time, valuable and reliable 
deductions will be made. 
In examining patients with the audiometer many curious effects are 
noticeable. The power of hearing is found to differ in both ears in 
nearly all persons; it varies with the height of the barometer, with the 
amount of air in the lungs, with the temperature, and many other as yet 
ill understood causes. These causes will soon be classified, and then the 
future of deaf people will have a brighter look than it has ever had, and 
the treatment of their diseases must then assume a more definite position 
in the world of medical science. Besides the treatment of deafness this 
instrument is invaluable in all examinations of men’s bodies for life insur- 
ance, the army, navy, telegraph service, railway servants, and others where 
good hearing as well as good sight is indispensable. Bya slight modifica- 
tion the audiometer can be converted into an electro inductive balance, 
and becomes available for the analysis of metals ; so delicate is it that it 
is said to have detected the 1,000th of a grain of silver in an alloy. 
Mr. W. R. Morris has exhibited this form of sonometer, as it ought 
to be called, to the society, when spurious coins were detected and the 
amount of waste due to wear and tear was shown upon the scale. It is 
difficult to say what uses this instrument will yet be put to, but it is 
clearly an exceedingly useful instrument for scientific research. 
