RELAY MAGNETS. 



centre of the disc towards its edge. In this case the style would 

 trace a spiral curve upon the paper, winding round it continually, 

 and at the same time retiring constantly hut slowly from its 

 centre towards its edge. 



172. Whichever method might be adopted, the paper would he 

 marked with a continuous succession of comhinations of lines of 

 varying lengths and dots, separated by spaces more or less wide. 

 These marks depending altogether on the succession of intervals 

 of suspension and transmission of the current, which intervals can 

 be varied and combined at will by an operator supplied with the 

 means of controlling the current which have been already ex- 

 plained, it will be easily conceived that an agent at s can trace 

 upon paper placed at s" in the manner here described such a 

 succession of characters composed of lines and dots as he may 

 desire ; and that an operator at s", being supplied with a key, 

 may interpret these characters, and thus translate the communi- 

 cation into ordinary language. 



It is also easy to conceive that the agent at s can stop'the clock- 

 work which moves the paper at s" or set it going at will, in the 

 same manner as he can ring a bell or discharge a cannon. 



173. It has been already explained that the intensity of the 

 current transmitted by a given voltaic battery along a wire of 

 given thickness must decrease in the same proportion as the 

 wire increases in length. -This loss of intensity due to the 

 length of the wire is increased in the practical operation of the 

 telegraphs by the loss of electricity arising from imperfect insu- 

 lation and other inevitable causes. It has therefore become a 

 matter of great practical importance to discover expedients by 

 which the intensity of the current may be re-established, or by 

 which the apparatus may be worked by a very feeble current. 



It was obvious that the intensity might be maintained at the 

 necessary degree of force by providing, as already stated, relay 

 batteries at intermediate stations sufficiently near each other to 

 prevent the current from being unduly enfeebled. But the main- 

 tenance of such numerous batteries in cases where great distances 

 must be traversed is expensive, and it was desirable to discover 

 some more economical expedient. 



174. The properties of the electro-magnet have supplied the 

 means of accomplishing this. 



The lever g h (fig. 58) may be constructed so light and so free, 

 that it will be capable of being moved by a current of extremely 

 feeble intensity. But if this lever were charged with any of the 

 functions by which it would become an instrument for giving 

 signals, such as the ringing of a bell or the motion of a style or 

 pencil, it would be necessary to impart to the electro-magnet and 



