560 Mr. L. Schwendler on a Method of using 



expressed as follows : — 



C = E \ — !- J- x 1000. 



E and K are two constants for any dynamo-electric machine; 



E is an electromotive force in volts ; 



K is of such dimension that v\/K represents an electrical 

 resistance ; 



m is the internal resistance of the dynamo-electric machine; 



r is the external resistance through which the useful work 

 by the main current has to be performed ; 



m and r are to be expressed in ohms. 



The resistance of the leading wires has been supposed nil. 



If we call R the resistance of a telegraph-line w r hich we 

 wish to feed from the main current, then the signalling-cur- 

 rent passing into that line when the main current is tapped 

 w r ill be 



E | l_ 6 K C+J | 1 



Jl-6 \r+m ) 1 



(. r-\-m j 



™ r 



R + r I r+m J R + r ' 



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

 less than 6 milli-oersteds. Hence w r e have the following 

 equation : — 



j l_g-<4i)% lOOOr c 

 E < > x ^ =6: 



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

 known. 



I need scarcely point out that, as R is invariably so large 

 that r can be negected 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 connexion with the 

 dynamo-electric machine. Hence the signalling through one 

 line is not influenced by the signalling on other lines; and in 

 this respect 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 r = 3, we have a main current of about 

 17,710 milli-oersteds; and the current passing into a line of 

 8000 resistance (800 miles of 5J w r ire) would be 6'6 milli- 

 oersteds. Supposing that all the fourteen lines at Calcutta 

 office are to be supplied with 6'6 milli-oersteds each, the cur- 



