1895-96.] Prof. M‘Kendrick on Electrical Musical Tones. 47 
thickness, so that a maximum amplitude of vibration may be 
obtained. The element of variable resistance is composed of a 
stationary electrode of carbon mounted in a case of insulating 
material. A diaphragm of carbon is fixed in the case. This 
forms the movable electrode of the apparatus, and between the 
stationary and movable electrodes is placed a quantity of carbon 
granules. 
When at rest, and before the key completing the circuit is closed, 
the iron diaphragm is held in tension by the permanent magnet. 
On closing the key, the coils of the electro-magnet energise and the 
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Plan of Graham’s Electrical Method of Producing Sound. 
diaphragm is attracted. This movement of the diaphragm operates 
through the air column in the flexible tube and the carbon dia- 
phragm is acted upon, thus increasing or diminishing the resistance 
of the circuit of variable resistance. Thus, for example, increase 
of the resistance decreases the strength of the battery current and 
causes a return movement of the receiver diaphragm, which, 
through the air column, produces a return movement of the 
diaphragm of variable resistance, causing a decrease in the resist- 
ance of the circuit. The consequent increase of the battery current, 
in turn, increases the energy of the receiver coils. Thus reciprocal 
effects are produced, resulting in the production of a musical 
