150 



ELECTRICAL MEASUREMENTS 



alternating-current voltage is then applied between the twc 



cylinders and the resulting current determined, p is calculated 



by aid of (1). 



Wire-wound Rheostats. For general laboratory purposes, a 



very convenient rheostat adapted to low voltages is shown in 



Fig. 77. 



The upright frame, 6 ft. high and 3 ft. wide, is strung with about 



550 ft. of bare /a/a wire, contained 

 in 100 sections. When 110 volts is 

 applied at the terminals of the frame, 

 one can, by means of spring clips, tap 

 off voltages or small currents. By 

 means of four clips and flexible con- 

 nection wires, the arrangement may 

 be divided into sections and these 

 connected in parallel. 



A cheap and convenient form of 

 rheostat, which has proved very use- 

 ful for loading the small generators 

 used for purposes of instruction in 

 electrical engineering laboratories is 

 shown in Fig. 78. 



The wire is wound in screw-threads 

 on moulded porcelain cylinders. 

 These cylinders are loosely held in 

 place on the angle-iron frame in such 

 a manner that they may be readily 

 removed. The base and the top, 



FIG. 77. Resistance frame. 



which carries the switches, are of " ebony asbestos wood, 

 following sizes of /a/a wire have been employed: 



Full load capacity 



The 



Size of wire 



No. 17 

 No. 20 

 No. 26 

 No. 23 



104 amp. at 110 volts 

 52 amp. at 110 volts 

 20 amp. at 220 volts 

 40 amp. at 220 volts 



Immersed-wire Rheostats. The carrying capacities of wires 

 may be greatly increased by immersing them in water, as will be 

 seen from the following table giving approximate data concerning 

 galvanized-iron wire. Immersed rheostats are useful in tempo- 



F^^^aECTR^' 



; FACULTY OF APPLIED SCIENCE. 



