2 



L. ScLwendler — Telerjraiily witli 'Fractioml CiirrenU. [No. 1, 



some useful mecLaiiieal work, i. e., pulling the punkhas, lifting messages, 

 producing a draught of cool air, &c. ; or the current may be made to pass 

 through a galvanoplastic apparatus in connection perhaps witli the Survey- 

 or General's Office, &e. 



Now connecting the negative pole* of such a dynamo-electric ma- 

 chine to earth, the positive pole to all the lines terminating in a telegraph 

 office, while the two poles are permanently connected by the resistance r 

 through which the current produces the useful work above-mentioned, then 

 it will be clear, without demonstration, that all the lines so connected can 

 be provided with signalling currents (whicb are exceedingly weak as com- 

 pared with the strong main current) by simply tapping the main current, 

 and that without perceptibly reducing it, i. e., without affecting the useful 

 %oorh performed by the main current through r. Supposing that the useful 

 work performed by the main current repays all the expenses connected 

 with the erection and working of the dynamo-electric machine, then 

 obviously this would be a method which would supply the signalling currents 

 for nothing. This might be an inducement for telegraph-administrations to 

 introduce the electric light, since they would get the signalling currents 

 into the bargain, and. the costly and cumbersome galvanic apparatus might 

 be dispensed with. 



An example will show this more clearly. A Siemens dynamo-electrio 

 machine of medium size can easily be made to produce through an 

 electric arc a current of 30,000 milli-oerstedts, of which not more than 

 3 milli-oerstedts are required to work the Siemens's ^^olarized relay with 

 engineering safety. Suppose that the sent current is made equal to 

 twice the current which is required to arrive, we have the following calcu- 

 lation for Calcutta office : — 14 long lines terminate at Calcutta, hence 14 x 

 6 = 84 milli-oerstedts would (as a maximum) have to be tapped off from 

 the main current of 30,000 milli-oerstedts. This represents a loss of only 

 0'28°/;, , — which is so small that not even the most sensitive eye would be 

 able to detect any variation in the light. 



Hence in this case we would feed the Telegraph lines with currents 

 which actually cost nothing, as the electric light alone would repay all 

 expenses. 



During my recent light experiments in London, it was exjjerimental- 

 ly established that the current in milli-oerstedts which a dynamo-electric 

 machine is able to jsroduce, can be exj^ressed as follows : — 



-^(— y 



, \/' + ;;;/ 



C = E ^ Ji-ZLf I X 1000 



r -f m 



* lu India we use positive signalling currents. 



