a small Fraction of a Dynamo-electric Current. 561 



rent carried off would be 6'6 x 14 = 92*4 milli-oersteds, or 0*5 

 per cent, 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, work- 

 ing through a given resistance r, could be even much greater 

 than 6'Q milli-oersteds. 



For any given R (resistance of the line) the current can 

 be increased by selecting a dynamo-electric machine with the 

 right internal resistance. 



The advantages of the method appeared to me sufficiently 

 great to justify a practical trial : — 



Experiment. October 11, 1879. — With a Siemens's dynamo- 

 electric machine (medium size) I produced a powerful electric 

 light ; and between the poles of the dynamo-electric machine 

 I connected up four artificial lines, each of 10,000 units resist- 

 ance, with relays ranging between 500 and 1000 units. These 

 four parallel circuits worked very well, singly and simulta- 

 neously. No variation of the electric light during telegraph- 

 ing could be noticed, even when the line resistance was re- 

 duced to 1000 units. Further, the resistance of one line was 

 increased to 20,000* and the signalling-currents were still 

 sufficiently strong (1*6 milli-oersted). 



Experiment, October 14, 1879. — Same as above ; but a branch 

 circuit was conveyed by the storeyard line (from the storeyard 

 where the dynamo -electric machine with its electric light was 

 put up) to Calcutta signalling-office (4 miles), and one of the 

 Agra lines (850 miles in length) worked by this current. 



The sent current at Calcutta was 9-6 milli-oersteds, the 

 received current at Agra 1*85. The great loss was due to the 

 exceedingly low insulation of the line near Calcutta. It is 

 now the breaking-up of the monsoons, when the climate in 

 Lower Bengal represents almost a hot vapour-bath. 



Several messages were sent to Agra, but no variation in the 

 electric light could be observed. 



Calcutta, October 17, 1879. 



