524 RHEOSTATS AND OTHER BALLAST [Cn. XIII 



tance in the circuit the less will be the amperage, and the less resis- 

 tance the higher the amperage. 



In some forms it is possible to have a great range of current, say 

 from 5 to 45 amperes (fig. 281); in other forms the range may be 

 limited, say from 1 5-2 5 amperes. 



For the projection microscope and the magic lantern it is desir- 

 able to have a rheostat giving a range of amperage from 5 to 25 



FIG. 281. THE USE OF AN ADJUSTABLE RHEOSTAT AS BALLAST FOR AN ARC 



LAMP 



G Generator (dynamo). 



A Arc lamp with right-angle carbons. 



AR Adjustable rheostat. 



5 If the movable contact-arm is at 5, the resistance allows but 5 amperes to 

 flow. 



25 If the contact-arm is at 25 then only half of the resistance is in the cir- 

 cuit and 25 amperes of current can flow. 



45 If the contact-arm is opposite 45, only a small amount of resistance is in 

 the circuit and forty-five amperes of current is allowed to flow. 



The arrow indicates the direction to turn the contact-arm to increase the 

 current. 



amperes. Such a rheostat is not difficult to construct, nor is it 

 expensive. The theater dimmer shown in fig. 183 is of this range. 

 But an adjustable rheostat requires judgment for its proper use; 

 for apparatus to be used by everybody it is better to have a fixed 

 rheostat ( 726). 



728. Installing the rheostat. It is usually placed close to the 

 arc lamp, i. e., inside the lamp switch, so that when the lamp switch 

 is open the current is entirely off the arc lamp and its rheostat. 



