Oct. 1st, 18S7.] 



SCIENTIFIC NE^VS. 



i«i 



transmitter, so-called from the fact that very small sounds 

 can be rendered audible by the use of a microphone and 

 telephone. The tread of a fly, for example, can be distinctly 

 heard if the insect is confined in a box placed on a sounding- 

 board or diaphragm to which the microphone is attached. 

 Professor Hughes invented the microphone in its present 

 form, though it is dependent on the principle involved in 

 Reis's transmitters. The Gower-Bell form of microphone 

 is shown in Figure 10, where SS' are two thin copper strips, 

 and CC are nine blocks, either of retort carbon or the carbon 



Fig. 10. The Gower-Bell Micropiioxe. 



rods used for electric lighting. The central block has eight 

 holes drilled in it, one opposite each of the outer blocks, 

 each of which has a similar hole drilled in it. Eight pieces 

 of carbon rods have their ends turned down as shown in 

 the figure, and these turned-down ends rest loosely in the 

 holes in the blocks. The current passes from one copper 

 strip to the other, through the carbons, blocks, and rods, 

 and has to go through sixteen loose contacts on its way, the 

 loose contacts being, as an electrician would say, four in 

 series and four in parallel, that is to say, the whole current 

 has four roads to split between, and each fractionof the current 

 passes through four loose ccntacts in succession. 



In this way the effect of the vibrations on the strength 

 of the current is multiplied. 



Fig. II is a section of the "Blake" transmitter. This is 



Fig. II. The Blakl Transihttek. 



probably the most widely used form of transmitter, and 

 bears a very strong resemblance to Reis's instruments. D 

 is the diaphragm, P a little platinum button on the end of 

 a very delicate spring, and C a hard carbon block attached 

 to a fairly stiff' spring. The current passes between the 

 platinum and the carbon. The two springs are carried by a 

 cast-iron bridge or bar, which is itself supported by a stiff 



spring at the same end, and has at theother extremity a screw 

 S banking against its inclined end. By turning this screw the 

 pressure between the contact points can be adjusted, and any 

 desired degree of sensitiveness obtained. This is an ex- 

 tremely good form of transmitter, though not so sensitive 

 as the microphone forms. It transmits with very clear 

 articulation, and can be adjusted, within limits of course, 

 for voices of varying loudness and pitch. Generally 

 speaking, a lighter pressure is best for high-pitched voices, 

 such as those of women and children ; a heavier pressure 

 is necessary if the instrument is to be used by men, whose 

 strong deep tones tend to set the springs into too great 

 vibration, causing complete interruptions of the current, 

 and producing an unintelligible growl or a succession of 

 sharp cracks in the distant receiver. 



There are otherforms of variable resistance transmitters in 

 use, notably Ader's, a microphone-form much in use on the 

 Continent, and Professor Silvanus Thompson's valve tele- 

 phone, which is a very peculiar form, consisting of a tiny 

 metallic or carbon ball standing on three little legs, at the 

 upper end of a tube leading from the mouthpiece, and 

 turning up by a gentle curve. The action is produced by 

 the air waves — somewhat concentrated, no doubt, by the 

 shape of the mouthpiece — passing up the tube and beating 

 on the lower surface of the ball, varying the pressure 



between it and its supports, and so varying the electrical 

 resistance between them. This transmitter speaks very 

 well, and it is unique among modern instruments in follow- 

 ing Reis's example, and sometimes using metallic surfaces. 

 Professor Thompson has, however, found some difficulty 

 in getting a metal free from the practical drawbacks 

 mentioned in describing Reis's instrument, and now uses 

 carbon contacts. He claims to have made a good working 

 transmitter, and it has already met with a certain amount 

 of commercial success. One form is illustrated in Fig. 1 j, 

 where A is the tube provided with a flaring mouthpiece, 

 V the valve, egg-shaped in this instrument, and P the 

 supports. The valve and supports are shown in plan to 

 the right of the general section. This is practically a 

 microphone with three contacts, but the current passes 

 from centre to circumference, or vice- versa; the coiled wire 

 connection to the valve can be seen attached to its upper 

 end. For further information on microphonic transmitters, 

 and some of the theories put forward in connection with 

 them, Professor Thompson's paper on Telephonic Investi- 



