CHAP. XIL] ELECTRICITY AND MAGNETISM. 403 



of the cable. The various means adopted to get rid of 

 this retardation are explained in Art. 275. 



431. Receiving Instruments for Cables. The 

 mirror-galvanometer of Sir W. Thomson (Art. 202) was 

 devised for cable signalling, the movements of the spot 

 of light sweeping over the scale to a short or a long 

 distance sufficing to signal the dots and dashes of the 

 Morse code. The Siphon Recorder of Sir W. Thomson 

 is an instrument which writes the signals upon a strip of 

 paper by the following ingenious means : The needle 

 part of a powerful and sensitive galvanometer is replaced 

 by a fine siphon of glass suspended by a silk fibre, one 

 end of which dips into an ink vessel. The ink is spurted 

 without friction upon a strip of paper (moved by clock- 

 work vertically past the siphon), the spurting being 

 accomplished electrically by charging the ink vessel by 

 a continuous electrophorus, which is itself worked by a 

 small electromagnetic engine. 



LESSON XL. Electric Bells, Clocks, and Telephones. 



432. Electric Bells. The common form of Electric 

 Bell or Trembler consists of an electromagnet, which 

 moves a hammer backward and forward by alternately 

 attracting and releasing it, so that it beats against a bell. 

 The arrangements of the instrument are shown in Fig. 

 165, in which E is the electromagnet and H the hammer. 

 A battery, consisting of one or two Leclanche* cells placed 

 at some convenient point of the circuit, provides a current 

 when required. By touching the " push " P, the circuit 

 is completed, and a current flows along the line and 

 round the coils of the electromagnet, which forthwith 

 attracts a small piece of soft iron attached to the lever, 

 which terminates in the hammer H. The lever is itself 

 included in the circuit, the current entering it above and 

 quitting it at C by a contact-breaker, consisting of a 

 spring tipped with platinum resting against the platinum 



