Unlike the hand telephone in every respect, the Blake transmitter consisted of a small black-walnut box, nearly square in form and having a funnel-shaped hole cut in the door to serve as a mouthpiece. Within the box was a soft iron diaphragm and suspended parallel to its center was a polished button of pure carbon; between the two hung a German-silver spring bearing a pellet of platinum which barely touched the center of the carbon. When the Blake transmitter was in use, the impinging sound waves pressed the diaphragm against the platinum and forced it with varying pressure against the carbon button. This changing pressure varied the resistance offered to the flow of the battery current, which pulsated through the carbon and into the primary winding of an induction coil or transformer, where it was converted into an alternating current through the inductive effects of the secondary winding and passed out in undulating or wavelike form into the line or subscriber circuit, thence through the copper wire in the green-covered telephone cord attached to the receiver, and on into the wire wound on the electro-magnets. Energizing the latter varied the attractive or pulling power of the pole pieces, thus causing the receiver diaphragm to vibrate in a manner exactly reproducing the vibratory motion of the transmitting diaphragm and setting up a series of sound waves in the receiver exactly corresponding to those produced by the vocal cords of the speaker.
So sensitive is a properly adjusted telephone diaphragm that its vibrations may cause several hundred thousand variations a minute in pressure of platinum point on carbon button in the Blake transmitter, or between carbon granules in certain other microphonic forms Xaturally the amount of current thus passing through this carbon gateway is extremely small, depending principally on the pitch of the speaker's tone and the physical condition of the line. Under ordinary circumstances and with both telephones and a complete copper circuit in good condition, distinct transmission of speech only requires a maximum generation of about one tenth of a milliampere of current at any one period, or only a millionth part of the current required to light an incandescent lamp. Again, probably only one fourth or less of even this infinitely small amount of current reaches the electro-magnets in the receiver, the other portion being used up in overcoming resistances. Where the circuit is three or four hundred miles in length, it is probable 'that only about one one-hundredth of the original current produced at the transmitting station is finally utilized at the receiving station.'
Where operating companies desired a less expensive instrument than the standard Blake set, for use of small users of service, only willing to pay a low rate, a much cheaper set (Fig. 33) was supplied. This set was originally intended to be used only on private lines, or
