528 RHEOSTATS AND OTHER BALLAST [CH. XIII 



In no case should one use naked wires for this rheostat, but the 

 rubber, water-proof insulated copper wires required by the National 

 Electric Code. The ends of the wires must be scraped and fastened 

 to the plates of iron. This is rather a poor make-shift for a rheo- 

 stat. The water soon heats up, and as it heats the resistance 

 becomes less so that more current flows. Then to counterbalance 

 this, fresh cold water can be added or the upper plate lifted to make 

 the distance between the iron plates greater. Furthermore, increas- 

 ing the amount of salt lessens the resistance. If there is too much 

 salt there will be too much current, if too little one cannot get 

 enough current without bringing the iron plates very close together, 

 and this is not safe. 



732. Home-made water cooled rheostat. A home-made 

 rheostat can be constructed of small, naked wire of the proper 

 length as shown by calculation or by the electrical tables. The 

 wire is wound around a wooden frame in a single layer, care being 

 taken that the different turns do not touch one another. The cut 

 ends of one of the heavy insulated supply wires are then soldered 

 to the two ends of the coil. The coil with the soldered junctions is 

 then immersed in a glass or porcelain dish containing pure water, 

 no salt being used (fig. 283). If the current is to be on for some 

 time it is a great advantage to have the vessel containing the rheo- 

 stat stand in a sink or in some place where water can drain away, 

 and then to keep a stream of cold water flowing into the vessel to 

 keep the wire cool. 



This general scheme is used in making tests of the gigantic 

 generators used in large power plants. For such tests the wire used 

 is naked telegraph wire of the right resistance and length laid out 



371a. With such a bucket rheostat, 12 liters (12 quarts) of l / 2 % salt 

 solution were used, and the distance between the iron discs could be as great 

 as 15 cm. (6 in.). With the discs 15 cm. apart and the solution at 23 centi- 

 grade a current of 10 amperes flowed. After an hour, when the temperature 

 had risen to 43 C., 12 amperes of current flowed. With the discs nearly in 

 contact 20 amperes were given. 



In this experiment the iron discs were 18 cm. (7 in.) in diameter. By in- 

 creasing the size of the iron discs the current could be increased, and by 

 diminishing it the current could be diminished. Iron (tin) funnels are some- 

 times used instead of discs. It is safer to have both discs covered with the 

 burlaps, and the conducting wires soldered to the discs or funnels. 



