238 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX 



FIG. 133. PROJECTION MICROSCOPE WITH AMPLIFIER. 



This picture shows the projection microscope arranged for use in a lecture 

 room. 



Commencing at the left : 



The supply wires to the table switch. 



A The ammeter to indicate the amount of current. It is along one wire 

 (in series). 



R The adjustable rheostat. It is along one wire. 



10-20 These figures indicate that the rheostat is adjustable; the lowest 

 current allowed to flow being 10 amperes and the highest 20 amperes. The 

 arrow indicates the direction to turn the knob to increase the current. 



The arc lamp in the lamp-house. This is the three- wire, automatic arc lamp 

 of the Bausch & Lomb Optical Co. 



The wiring is shown to be: 



One wire from the negative pole of the switch to the pole for the lower carbon. 



One wire passes from the positive pole of the switch to the middle binding 

 post of the motor mechanism of the automatic lamp. The current for the 

 motor does not traverse the rheostat. 



One wire passes from the positive pole of the switch to the ammeter, to the 

 rheostat and from the rheostat to the positive ( + ) binding screw of the art- 

 lamp. 



