CH. XIII] 



RHEOSTATS AND OTHER BALLAST 



531 



rheostats. According to Ohm's 

 law ( 724a), the amount of cur- 

 rent varies inversely as the resis- 

 tance, then if two equal rheo- 

 stats were used only half as 

 much current would flow as FlG 28? RHEO STATS IN SERIES. 

 when one rheostat is used. Also c Dynamo, 

 if the voltage is increased the A Arc lamp. 



i-, ,-, R,, R, Rheostats in series, all the 



amperage will increase in the cun4nt must pass through both of them 

 same ratio provided the resis- (compare fig. 288). 



mi The two rheostats R, and R, are con- 



tance remains constant. Then nec ted in series to get a smaller current 

 if one has two rheostats, each than can be obtained by the use of one 

 of the right capacity for an arc 



lamp with a 1 10 volt circuit, the two in series would give approxi- 

 mately the correct number of amperes on a 220 volt circuit. The 

 amperage would be somewhat higher on the 220 volt circuit because 

 when used singly on a no volt circuit each is somewhat reinforced 

 by the resistance of the arc lamp. When both are used for one 

 lamp on a 220 volt circuit there is not twice the resistance, hence 

 the amperage will be somewhat greater than with one rhostat on 

 a no volt circuit. 



735. Rheostats in parallel. 



parallel as shown in fig. 288, two 



FIG. 288. Two RHEOSTATS IN PAR- 

 ALLEL, GIVING Two PATHS 

 FOR THE CURRENT. 



G Dynamo. 



A Arc lamp. 



R It R 2 Rheostats in parallel. 



With two or more paths for the cur- 

 rent, the total amperage will be the 

 sum of the amperages going over each 

 path ( 735). 



If two rheostats are inserted in 

 paths are furnished for the cur- 

 rent. The amperage given by 

 both will be the sum of that given 

 by each separately, for example, 

 if one had two fixed rheostats, 

 each one giving five amperes of 

 current, if they were connected 

 with the line in parallel, 10 am- 

 peres would be allowed to flow. 

 On the other hand if they were 

 connected in series (fig. 287) so 

 that all the current had to flow 

 through both of them then only 

 2^2 amperes of current would be 

 available. (See 724 a). 



