H> Prof. Hughes on the Physical 



when converted into the peculiar charcoal I have discovered 

 ami described. 



It is not only necessary to vary the number of surfaces and 

 materials in accordance with the range and power of the vibra- 

 tions, but these surfaces and materials must be put under more 

 or less pressure in accordance with the force of the sonorous 

 vibrations. Thus for a man's voice the surfaces must be 

 under a far greater pressure than for the movements of insects. 

 IS till the range of useful effect is very great, as the boxes 

 which 1 have specially arranged for man's voice are still sen- 

 sitive to the tick of a watch. 



In all cases it should be so arranged that a perfect undu- 

 latory current is obtained from the sonorous vibrations of a 

 certain range. Thus, when speaking to a microphone trans- 

 mitter of human speech, a galvanometer should be placed in 

 the circuit, and, while speaking, the needle should not be de- 

 flected, as the waves of + and — electricity are equal, and 

 are too rapid to disturb the needle, which can only indicate a 

 general weakening or strengthening of the current. If the 

 pressure on the materials is not sufficient, we shall have a con- 

 stant succession of interruptions of contact, and the galvano- 

 meter-needle will indicate the fact. If the pressure on the 

 materials is gradually increased, the tones will be loud but 

 wanting in distinctness, the galvanometer indicating interrup- 

 tions; as the pressure is still increased, the tone becomes 

 clearer, and the galvanometer will be stationary when a maxi- 

 mum of loudness and clearness is attained. If the pressure be 

 further increased, the sounds become weaker though very 

 clear ; and as the pressure is still further augmented the 

 sounds die out (as if the speaker were talking and walking 

 away at the same time) until a point is arrived at where there 

 is complete silence. 



When the microphone is fixed to a resonant board, the lower 

 contact should be fixed to the board, so that the sonorous vi- 

 brations act directly on it. The upper contact, where the 

 pressure is applied, should be as free as possible from the in- 

 fluence of the vibrations, except those directly transmitted to 

 it by the surfaces underneath ; it (the upper surface) should 

 have its inertia supplemented by that of a balanced weight. 

 This inertia I find necessary to keep the contact unbroken by- 

 powerful vibrations. No spring can supply the required 

 inertia ; but an adjustable spring may be used to ensure that 

 the comparatively heavy lever shall duly press on the contacts. 



The superposed surfaces in contact may be screwed down 

 by an insulated screw passing through them all, thus doing 

 away with the lever and spring ; but this arrangement is far 



