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the principle, serves well on lines of moderate length, if kept in good adjustment. Fig. 10 shows the side and back of the transmitter at A and B, respectively. The carbon is in the form of a button r and is mounted in a case m supported by a steel spring s. A platinum ball c is supported between the carbon button r and the diaphragm n, the

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diaphragm being held against the receptacle 1l and insulated therefrom by a hard-rubber circular sleeve ee. The platinum ball or contact piece is held rigidly by a German-silver spring a. The springs s and a are insulated from each other at i and both are supported by y, a heavy metal strip, which in turn is fastened to Il by the spring k. The pressure between the platinum contact, the carbon button, and the iron diaphragm is adjusted

by varying the position of the strip y by means of the screw W. The two metal strips p and f are used, the former to hold the sleeve ee in position, and the latter as a damper to check the vibrations of the diaphragm as soon as they have served their purpose so that they will not interfere with those following. For this reason, the end of the spring f, which presses upon the diaphragm near its center, is covered with a small cloth pad x. The entire apparatus is mounted in a wooden box dd, into which is fitted a wooden mouthpiece v.

Current entering the transmitter at the binding screw o passes down the spring a and ·through the platinum ball and carbon button to the spring s, thence through the spring k to the binding screw u and out. The vibrations of the diaphragm vary the pressure between the carbon button and the platinum contact, causing the necessary change of resistance in the transmitter circuit. The Blake transmitter, although low in first cost and requiring but little current to operate it, needs frequent and careful adjustments, and is so very sensitive to vibrations that special care is necessary to select for it a particularly firm support.

The Solid-Back Transmitter employs granulated carbon for the active material and is the one in most common use to-day. Fig. II shows a side view and a back view of this transmitter at A and B, respectively. The outer case s is of metal, shaped much in the form of an electric bell gong. To this is attached a metal cover rr

containing the mouthpiece. The granulated carbon can be seen at c inclosed in a brass chamber mn, made in two parts, which are screwed together. This chamber is supported by the collar and screw at e to the brass strip k, which, in turn, is screwed to the cover rr. On the inner sides of the chamber are two carbon blocks a and v, the latter together with a mica washer ii forming one side of the chamber, and the former a portion of the other


Fig. 11.—The Solid-Back Transmitter

side. These blocks are electroplated and soldered to their respective holders. The remaining parts of the inner walls of the chamber mn are lined with varnished paper. The support of the carbon block v consists of a disk-shaped piece of brass o, which, by means of the bolty, is held firmly against the diaphragm b. The diaphragm is usually made of aluminum, thoroughly varnished, about 21 inches in diameter and 0.022 inch thick. It is fastened to the cover r by the hard-rubber circular sleeve uu which overlaps it 1 inch, and is damped by the cloth-covered end of the spring g. The sleeve u is held in place by the clip p.

Connection with this transmitter is made by inserting the solid tip of an insulated connecting cord in the copper-faced hole d, which is insulated from the outer case by a hard-rubber bushing. A fine wire leads from here to o, and the circuit continues through the carbon plates and carbon granules to e, thence by the strip k to the outer metal case, and leaves the instrument by means of a connection to the metal arm l of the transmitter. The vibrations of the diaphragm are readily transmitted to the carbon block v, the mica washer being sufficiently elastic to permit of this, and the pressure on the carbon granules c varies accordingly, providing the necessary variations of resistance in the transmitter circuit. This variation is from about 35 ohms to 75 ohms.

The solid-back transmitter leaves little to be desired; it is sensitive to sound waves, but not to mechanical vibrations; the carbon blocks secure excellent contact with the carbon granules, and the latter give but little trouble by caking or“packing” because they do not become much heated by the current, owing to there being space above and below the blocks in which they can expand; the transmitter case is very small; and the chamber containing the carbon granules is both air-light and moisture-proof. When properly set up, the transmitter requires no adjustment. In case it is necessary to examine the working parts, the insulated connecting cord must first be withdrawn from the copper-faced hole d; then the screws which hold the cover to the case must be taken out, after which the working parts can be withdrawn.

The Induction Coil.–An induction coil is used in Fig. 8 for several reasons. This instrument, by providing a short local circuit for the transmitter entirely independent of the line circuit makes the variations in the resistance of the carbon large in comparison with the total resistance of the circuit in which it is connected, and therefore makes the action of the transmitter more effective. It also decreases the resistance of the line circuit by an amount equal to that of the local circuit, and by changing the pulsating current into an alternating one makes it more effective in exciting the diaphragm of the receiver. Another reason is its ability to alter the current and pressure relations of the primary and secondary circuits. This variation is proportional to the ratio of the number of turns in the primary winding Þ to the number of turns in the secondary winding s. Thus, with 300 turns in the primary and 2,400 turns in the secondary, the ratio is I to 8. A pressure of E volts (see Appendix) across the primary winding would, in this case, be raised to 8 E volts at the terminals of the secondary winding, while the primary circuit of I ampere would be reduced to s ampere in the secondary or line circuit. The electric energy in both circuits, however, remains the same, namely, EI. As the resistance losses on the line are pro

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