44 



THE POPULAR SCIENCE MONTHLY. 



railway, with the view of ascertaining whether the rails could be em- 

 ployed as lines of telegraph, made the discovery that the earth would 

 serve instead of a return- wire, and with the advantage of diminished 

 resistance ; the earth, in fact, behaving like a return-wire of infinitely 

 great cross-section, and therefore of no resistance. 



We are not, however, to suppose that the current really returns 

 from the receiving to the transmitting station through the earth. The 

 duty actually performed by the earth consists in draining off the op- 



Fig. 2. 



Fig. 3. 



Single-needle Instrument. 



Internal Arrangements. 



posite electricities which would otherwise accumulate in the terminals. 

 It keeps the two terminals at the same potential; and, as long as this 

 condition is fulfilled, the current will have the same strength as if the 

 terminals were in actual contact. 



One of the best-known telegraphs in England, though little or not at 

 all employed elsewhere, is the single-needle instrument of Wheatstcnf 

 and Cooke, represented in Figs. 2 and 3, the former showing its exter- 

 nal appearance, and the latter its internal arrangements as seen from 

 behind. The needle, which is visible in front, is one of an astatic pair, 

 its fellow being in the centre of the coil (C C). When the handle (H) 

 hangs straight down, the instrument is in the position for receiving 

 signals from another station. The current from the line- wire enters at 

 L, and, after traversing the coil and deflecting the needle, escapes 

 through the earth-wire (E), having taken in its course the two tall 

 contact-springs (t t'). 



