ELECTRICAL MACHINERY 493 



A 16 C.P. lamp requiring 55 watts on a 5O-volt circuit 

 will take about one ampere ; on a i lo-volt circuit it will 

 take 0.5 ampere ; on a 22O-volt circuit about 0.25 ampere. 

 A lamp should not be subjecteti to a voltage higher than 

 its rating; the filament is not made for it and will soon 

 give out. 



The efficiency of a lamp is proportional to the ratio of 

 the number of candles it will produce to the number of 

 watts it absorbs. A high efficiency is 3 watts per candle 

 power, and the average efficiency is 3.5 watts candle power. 

 High-efficiency lamps are used where the pressure is very 

 closely regulated or cost of power is high, and low- 

 efficiency lamps are used where there is not such close 

 regulation and power is less expensive. 



664. Potential distribution in lamp circuits. Incandes- 

 cent lamps are usually operated from low-voltage con- 

 stant-potential circuits. Where lamps are supplied with 

 current from a street car circuit, which generally has a 

 potential of 500 volts, they are grouped in multiple series ; 

 i.e., 5 loo-volt lamps or 10 5o-volt lamps will be connected 

 across the mains. In a series circuit the drop on the lead 

 wires does not interfere with the regulation of the vol- 

 tage at the terminals, but in a parallel circuit this drop is 

 an important factor and requires that the lamps be dis- 

 tributed and the size of wire proportioned so that each 

 lamp receives about the same voltage. For example, con- 

 sider loo 22O-volt lamps to be connected at distances 

 along a pair of mains which extend 500 feet from a gen- 

 erator which has a potential difference of 225 volts at the 

 brushes. The lamps nearest the dynamo will receive a 

 greater potential than their rated capacity and will often 

 burn out, while those farthest from the dynamo will not 

 receive potential equal to their capacity, hence will burn 

 dimlv. In order to overcome this, centers of distribution 



