of Edinburgh t Session 187 7—7 8. 
619 
axis (fig. 4), where A is silent when its prongs are equidistant 
from the middle of the coil, and the fork B loudest. All this 
is, as we have said, simply an illustration of a well-known 
action, and at the same time a beautiful demonstration of the 
way in which a tuning-fork vibrates. The coils I used were of 
fine copper wire, 1^ inch diameter and J thick, but smaller coils 
would do equally well, and the forks were the ordinary small A 
and C forks sold by the musicsellers. It is perhaps worthy of 
note that a coil, a magnetised fork, and a telephone form a handy 
combination for testing the completeness of a circuit, as the sound 
of the fork coming directly to the ear is immensely below that 
heard in the telephone in the operator’s hand. When the telephone 
does not sound, there is a break in the circuit. In these various 
performances of the fork, we have evidence enough to prove that 
the cause assigned by Bell for the sending action of the telephone 
covers at least the greater part of that action. At the same time, it 
must be borne in mind that the vibrations of the fork, and the 
sounds produced by them, are immensely greater than any connected 
with the telephonic effect of the voice, and that it is possible that 
the conditions of iron vibrating under the energy treasured up in 
it may be different from what they may be when the iron is beaten 
by the air. 
But even this tuning-fork performance is not quite free from 
ambiguity. To find whether there might not be some change of 
magnetic condition due to internal 
vibration, able to generate currents, 
I cemented two coils (fig. 5.) to the 
vibrating ends of a large tuning-fork. 
It was a C (256 vibrations), with 
prongs upwards of 6 inches long, 
broad, and more in average than 
5 thick. The distance between the 
prongs at the end inside was inch. 
The coils weighed \ oz. ; they were 
f inch diameter, 1 inch long, and 
were of *007 inch copper wire. 
They reduced the pitch from C to 
A. The cement was the hard and tough black tarry compound 
