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Page:Popular Science Monthly Volume 70.djvu/410

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406
THE POPULAR SCIENCE MONTHLY

bell box so that the battery circuit might be under the control of and closed and opened by the telephone hook switch. Interior and sectional views of the Blake transmitter are shown in Fig. 31. With each of these early Blake instruments a circular was sent stating that it

can be used only as a transmitter, and requires a telephone to hear with. This is the most complete and perfect set of instruments that can be used for telephonic communication. It will transmit the faintest whisper with perfect distinctness.

It is a fact that no modern transmitter exceeds the Blake in clearly and distinctly reproducing the articulation of the subscriber. But owing to the mechanism employed in its single contact form, it proved deficient in volume or power required on noisy and on long suburban lines. Again, its first cost was comparatively low, and the Blake and similar types of transmitters possess the striking advantage over the old hand telephone of being placed on a local circuit, thus removing their varying resistances from the line circuit, to the improvement of the qualities of transmission. The old hand telephone and the early box magneto telephone formed a part of the main-line circuit, thus materially increasing its resistance.

The first Blake instruments were larger in whole and in part than the transmitter so familiar to all telephone users, while the screw that controlled the proper adjustment of the electrodes projected through the box, thus making it possible for the subscriber to adjust the instrument for long circuits or short lines, regardless of the mood he might be in. It only required a little experience to teach the local companies that the wiser plan was to have trained telephone inspectors do the necessary adjusting. So the adjusting screw was put inside the box and the door fastened with lock and key.e

There is a wide difference between the underlying principles of Bell's self-contained transmitter and his variable resistance transmitter, both of which were exhibited at the Centennial. The microphone, or carbon, or battery transmitter, now in use on nearly all telephone lines, belongs to the variable resistance type. Unlike the early hand transmitter, it does not generate current, but serves as a voice-governed mechanical regulator of the flow of current chemically generated in a battery.

After Graham Bell had shown how to solve the problem of speech transmission, many other inventors were naturally quick to suggest commercial improvements. A few worked hand-in-hand with Graham Bell and gladly contributed to his success. Among this number was Francis Blake, Jr., who invented the transmitter bearing his name and which was the only transmitter used on a majority of the Bell lines prior to 1893. Mr. Blake was a Christmas present in 1850; graduated from the Brookline, Mass., High School in 1866; entered