530 



RHEOSTATS AND OTHER BALLAST 



[Cn. XIII 







FIG. 285. To SHOW THE TIN PLATE CUT 

 IN INCOMPLETE STRIPS FOR THE 

 RHEOSTAT. 



Cut in this way the tin plate is like a 

 continuous flat wire. 



straight in the bottom of a 

 river or creek. The flowing 

 water keeps the resistance 

 wire cool. 



733. Home-made rheo- 

 stat of tin strips. A good 

 adjustable rheostat for experi- 

 mental purposes can be cheap- 

 ly made by cutting tinned 

 sheet iron into strips as shown 

 in figure 284, 285, and nail- 

 ing these strips to a wooden 

 frame. One end of the con- 

 ductor is fastened to one end 

 of the sheet, and the other to 

 the other end of the sheet. 



To make this an adjustable 

 of heavy copper wire or of sheet copper 

 to the other as shown. By this 



rheostat, a "juniper 

 is put across from one sheet 

 means the current can be sent through as much or as little of 

 the resistance as desired, thus giving a great range in the 

 amperage. As the surface is very great in the thin sheet iron, the 

 air currents carry off the heat developed so that this rheostat does 

 not become unduly heated. It is a very common form around 

 physical laboratories, but is bulky and not very well adapted to a 

 magic lantern or a moving picture installation. Furthermore, such 

 a rheostat does not fulfill the requirements of the National Elec- 

 trical Code, as there is too much 

 combustible material in connec- 

 tion with it, and the resistance 

 is not boxed in. 



734. Rheostats in series. 

 If one has two rheostats, less 

 current will be allowed to flow FIG. 286. 

 if they are connected to the line 

 in series, that is, so that all the 



AN ELECTRIC CIRCUIT AND 

 GENERATOR. 



C Generator. 

 A Arc Lamp, 

 current must flow through both R Rheostat. 



