Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
[Ch. 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 s to 45 amperes (fig. 281); in other forms the range may be 
limited, say from 15-25 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 ax Arc 
G Generator (dynamo). 
A Are lamp with right-angle carbons. 
AR Adjustable rheostat. 
5 If the movable contact-arm is at 5, the resistance allows but 5 amperes to 
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 
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 swatch 
is open the current is entirely off the arc lamp and its rheostat.


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