NO. 2 



HISTORY OF ELECTRIC LIGHT SCIIROEUER 



37 



one end of each pair of opposite coils being connected together and 

 the other ends connected to a commntator segment. Thus the arma- 

 ture itself was not a closed circuit. The machine had two pairs of 

 horizontal poles between which the coils revolved. One end of the 

 one pair of coils in the most active position was connected, by means 

 of two of the four brushes, in series with one end of the two pairs 

 of coils in the lesser active position. The latter two pairs of coils 



Brush's Arc Lamp, 1877. 

 The carbons were differentially controlled. This lamp was used 



for many years. 

 Institution. 



This lamp is in the collection of the Smithsonian 



were connected in multiple with each other by means of the brushes 

 touching adjacent commutator segments. The outside circuit was 

 connected to the other two brushes, one of which was connected to 

 the other end of the most active pair of coils. The other brush was 

 connected to the other end of the two lesser active pairs of coils. The 

 one pair of coils in the least active position was out of circuit. The 

 field coils were connected in series with the outside circuit. 



Brush's arc lamp was also dififerentially controlled. It was de- 

 signed for about 10 amperes at about 45 volts. The carbons were 

 copper plated to increase their conductivity. Two pairs of carbons 

 were used for all-night service, each pair lasting about eight hours. 



