ELECTRIC DISTRIBUTION AND WIRING. 267 



connected in series, and these groups of lamps are connected in 

 parallel with each other across the mains. This arrangement is 

 exemplified in the lighting of electric cars where the standard sup- 

 ply-voltage is 550 volts and where the lamps for lighting the cars 

 are usually iio-volt lamps connected in series-groups of five 

 lamps each, these groups being connected in parallel with each 

 other between the trolley and the rail. A similar arrangement is 

 employed for the very low voltage osmium lamps * which are 

 now coming into extensive use abroad. Thus three 3/-volt 

 osmium lamps are connected in a series-group and such groups 

 are connected in parallel with each other across standard i lo-volt 

 mains. 



(b) The connection of parallel-groups of lamps in series. In the 

 early days of electric lighting the constant-current method of 

 supplying arc lamps for street lighting was quite common. Many 

 towns which were provided with this series-system of distribution 

 were not provided with any other means for supplying incandes- 

 cent lamps, and the only feasible method for operating incandes- 

 cent lamps was to connect a group of such lamps in parallel 

 and to connect this group in series in the arc-lamp circuit. This 

 arrangement is now seldom or never used. 



Advantages and disadvantages of the connection of series -groups 

 of lamps in parallel. In order to clearly understand the advan- 

 tages of grouping electric lamps in series it is necessary to keep 

 in mind the fact that in the earlier days of electric-lighting elec- 

 tric lamps could not be made satisfactorily for higher voltages 

 than about 100 or no volts ; and' that, at the present time, the 

 higher-voltage carbon-filament lamps are less efficient or shorter 

 lived than low-voltage lamps because of the excessively small 

 size of the filament of a high-voltage lamp. This may be summed 

 up in the general statement that the electric lamp is essentially a 

 low-voltage device, so that if one wishes to use a high voltage in 

 order to reduce the amount of copper required for a given installa- 

 tion the lamps must be arranged in series-groups. The saving 



* See London Electrician, Vol. 55, p. 141, May 12, 1905. 



