Fig. 172
The plate a b is inserted in the line, and the gap between this and the plate c offers sufficient resistance so that the telephone circuit suffers no leakage at this point, but lightning has such extremely high tension that it readily passes across this gap and finds its way to the earth without damaging the instruments.
We have already noticed that our alternating current dynamo, which produces 60 vibrations per second in the telephone receiver, causes it to give a tone very nearly like the C, which is two octaves below middle C upon the piano. C requires 64 vibrations per second. We may speed up our dynamo so as to make it yield a tone exactly like C or even above it.
Dr. Cahill of Holyoke, Mass., has devised an organ in which alternating current dynamos produce the necessary number of vibrations for each tone. The name telharmonium has been proposed for this organ. It has a separate dynamo for each tone, each dynamo having a frequency corresponding to the tone required of it. The dynamo, for instance, which produces middle C makes the electric currents surge back and forth 256 times a second, and this causes the diaphragm of a telephone receiver to vibrate 256 times a second, and this sends forth 256 air waves per second, and when these reach our ears we recognize the tone we call middle C. The frequency of alternation in a dynamo may be increased by either increasing its speed of revolution or by increasing the number of coils upon its armature.
Mr. Cahill's great organ looks like a large machine shop with many counter shafts geared so as to run at different speeds. On each shaft are a large number of little dynamos whose armatures have various numbers of coils. The organist, who may be far removed from this "machine shop," fingers an ordinary keyboard. Each key opens and closes a switch, thus bringing into action its own dynamo.
If the key which is known as C, one octave below middle C, is pressed down, a switch closes the circuit between the telephone and a dynamo which gives 128 double alternations of current.
The tone which is produced by 128 vibrations per second is the one most often heard from a man's voice in ordinary conversation.
Another key brings into action upon the same telephone receiver—and at the same time if desired—a dynamo which gives twice as many alternations per second and produces the tone most often heard in female conversation. It is middle C.