RESISTANCE OF EXTERNAL CIRCUITS. 1 21 



had the minimum resistance. But as, in order to produce 

 the electric light, a part of this external circuit must have a 

 certain resistance, this resistance becomes useful ; and we 

 have seen that, according to Joule's law and also according 

 to the experiments of Jamin and Becquerel, the maximum 

 effect is obtained when this useful resistance is equal to the 

 useless resistance, plus that of the generator. It is, there- 

 fore, the resistance of the arc which has to be especially 

 considered from this point of view : and it will always be 

 advantageous to diminish as much as possible the resistance 

 of the conductors used for simply conveying the electricity. 

 Under these conditions the loss to which the light is liable 

 may take place in greater or less proportion, according to 

 the resistance of the generator. This is to a certain extent 

 explained by the circumstance that, the resistance of the 

 conductors being added to that of the generator, the former 

 tells more in the final result in proportion as the latter is 

 smaller. It must, besides, be remembered that the increase 

 of the unutilized resistance of the circuit, by diminishing the 

 intensity of the electric current, diminishes the energy of the 

 mechanical resistance opposed to the motor, and by allowing 

 it to acquire greater speed renders the loss of electric inten- 

 sity less marked. The reverse will, of course, occur when 

 the resistance of the circuit diminishes instead of increasing. 



The following results of some experiments made at the 

 South Foreland will illustrate these remarks. Between the 

 lighthouse and the building containing the machines, a dis- 

 tance of 694 feet, there were carried for the electric light 

 three cables, two of which were each formed of seven copper 

 wires No. 14 (Birmingham Wire Gauge), and were joined to 

 constitute the circuit, thus giving a total length of 1,286 feet, 

 with a resistance of 0*32 of Siemens' unit, or about that of 

 33 metres of telegraph wire. 



With Holmes' machine, which had the greatest resistance, 

 the loss of luminous intensity was estimated at i6'i per cent. 

 with the Gramme machine, of much less resistance, it was 



