1880.] L. Scbwendler — Telegraphy loith Fractional Currents. 



3 



E and k are two constants for any dynamo-electric machine. E is an 

 electromotive force in volts ; k is of such dimensions that « ■v/ k repre- 

 sents an electrical resistance ; in is the internal resistance of the dynamo- 

 electric machine ; r is the external resistance through vyhich the iisef ul 

 work by the main current has to be performed. 



m and r are to be expressed in ohms. The resistance of the leading 

 vfires has been supposed nil. 



If we call E. the resistance of a telegraph line, which we wish to feed 

 from the main current, then the signalling current passing into that line 

 when the main current is tapped would be 



Gr \ ^ — e + '"'^ ) 1000 r 



— E 



K + / r + m \ U + r 



and this current, in the case of the Indian lines, should not be less than 6 

 milli-oerstedts. Hence we have the following equation : — 



/ + ) ii + r 



from which r can be calculated, since E, K, m, v and li are known. 



I need scarcely point out, that as li is invariably so large that r can 

 be neglected in comparison with it ; the current in one line only depends on 

 the resistance of that line, and not on the resistance of the other lines in con- 

 nection with the dynamo-electric machine. Hence the signalling through 

 one line is not influenced by the signalling on other lines ; and in this re- 

 spect the method is on a par with signalling through different lines by 

 separate batteries. 



We will take a special case. — For a Siemens's medium machine, making 

 }• = 3, we have a main current of about 17,710 milli-oerstedts, and the 

 current passing into a line of 8000 resistance (800 miles of 5-|- wire) would 

 be 6'6 milli-oerstedts. Supposing that all the 14 lines at Calcutta office 

 are to be supplied with 6"6 milli-oerstedts each, the current carried of£ 

 would be 6'G x 14 = 92-4 milli-oerstedts, or 0-5 "/o of the main current. 



It is best to make all the lines equal in resistance by adding to the 

 shorter lines some artificial resistance. This measure would prevent a dead 

 earth (occurring on one of the lines and close to Calcutta) from having any 

 effect on the working of the other lines. In Europe, where the lines are 

 much shorter, the signalling currents supplied by a given dynamo-electric 

 machine, working through a given resistance r, could be much greater than 

 C'G milli-oerstedts. 



