94 SCIENCE PROGRESS 



yards behind the most advanced trenches. Close to the Hne 

 and in front of the microphone base was a dug-out, known 

 as the observation post ; this was also connected electrically 

 to H.Q. Two trained observers were stationed there, and 

 they controlled the recording apparatus at H.Q. They were 

 sufficiently in advance of the microphone base to enable 

 them to hear the report before it actually reached the micro- 

 phones, and to start the film running by the time the micro- 

 phones were affected. 



To understand the method of working we must consider 

 a most beautiful device in electricity known as the Wheat- 

 stone Bridge. This is, in effect, simply an electrical balance ; 

 it consists essentially of four arms containing resistances, the 

 slightest variation in which can be detected by a sensitive 

 galvanometer, which takes the place, in this electrical arrange- 

 ment, of the pointer in the ordinary balances used for weighing. 

 If the resistances in the four arms be P, Q, R, and S, respectively, 

 then it can be shown that when they are adjusted in such a 

 way that P '. Q = R : S, then no current at all will flow through 

 the galvanometer, since perfect balance is attained. 



If, however, the resistances are altered so that this relation 

 is no longer true, then a current will flow and the needle of 

 the galvanometer will be deflected. 



In sound-ranging there were six distinct Wheatstone Bridge 

 arrangements, one for each microphone. The microphone 

 in question, together with the wire connecting it to H.Q., often 

 some miles in length, was made to correspond to one resistance, 

 P, whilst the other three, Q, R, and S, were at H.Q., and were 

 adjusted to complete the required balance. 



In practice it was found that the microphone took an 

 appreciable time to heat when the current was switched on, 

 so that the balance of the bridge was upset. This difficulty 

 was met by putting a second microphone in the second arm 

 of the Bridge, so that its fluctuations should correspond more 

 or less to those of the microphone in the field. 



The galvanometer then remained appreciably steady, and 

 gave a continuous line as its record on the film. 



But when the gun report reached the recording microphones 

 it completely altered their resistance, thus causing a current 

 suddenly to flow through the galvanometer, the string of which 

 gave a sharp kick, which in turn was photographed as a break 

 in the otherwise continuous line. (See Fig. i.) 



We have now to consider the apparatus. A small electric 

 lamp was so arranged that it sent a beam of light through a 

 case containing the time-wheel on to a galvanometer, the re- 

 cording part of which consisted of six vertical strings. The 

 shadows of these strings cast by the beam of light were re- 



