Page:Popular Science Monthly Volume 14.djvu/145

Rh Fig. 12 illustrates a microphone having ten plates of silk, a mixture of dextrine and lampblack having been previously worked into the pores.

In Fig. 13 fifty disks, D, of protoxide of iron, are shown inclosed in a glass tube.

A novel form of transmitter, used by Mr. Edison in his experiments, is shown in Fig. 14. The semi-conductor is a collection of small fragments



of cork, covered with plumbago. It can be used with or without a diaphragm.

The instrument shown in Fig. 15 acts both as a transmitter and receiver. The solid carbon of the transmitter is here replaced by silk fibres coated with graphite. Its action as a receiver is probably due to the attraction of parallel currents; the volume of the whole being contracted during the passage of a current through F.

In the accounts which have been published of experiments with the microphone, the statement has frequently been made. that minute sounds are actually magnified by it, in the same sense that minute objects are magnified by the microscope. A little reflection will show, however, that there is no real analogy in the action of the two instruments. The sound that is heard in the receiving-instrument of the microphone, when a fly is walking across the board on which the transmitter is placed, is not the sound of the fly's footsteps, any more than the stroke of a powerful electric bell, or sounder, is the magnified sound of the operator's fingers tapping lightly, and it may be inaudibly, upon the key. This view of the subject readily explains why the microphone has failed to realize the expectations of many persons, who, upon its first exhibition, enthusiastically announced that by its aid we should be able to hear many sounds in Nature which had hitherto remained wholly inaudible.

—a number of the telephones invented by Mr. Edison may be classed together as short-circuiting or