CH. IXa\ 



PROJECTION MICROSCOPE 



253 



the lamp mechanism. If the carbons do not separate when an automatic 

 lamp starts it is due to too small an amount of current. If the current 

 is too weak the magnet is not strong enough to lift the carbon. 



FIG. 206. Diagram showing proper 

 wiring for a projection apparatus. 



i -f- Main wire conducting the cur- 

 rent to the apparatus ; 2 , Main wire 

 conducting the current away from the 

 apparatus ; j, resistance or rheostat. It 

 should be adjustable to allow a greater or 

 less amount of current to pass as occasion 

 demands, 



4. Ammeter to show the amount of 

 current. This is very desirable, but not 

 absolutely necessary. If it is not used the 

 wire would take the course indicated by 

 the dotted line. 



5. Arc lamp. The left or upper 

 carbon marked with the arrow and plus 

 sign ( -j- ) is the one in which is fouud the 

 crater giving the illumination ; 6, Arc 

 lamp similar to 5. 



7. Lever of the double pole, double 

 throw switch. The conducting wires go 

 to the binding posts at the base of the 

 lever. In case the crater does not appear 

 in the upper carbon, but in the one mark- 

 ed with the minus sign ( ), the conduct- 

 ing wires i and 2 are interchanged and 

 must be reversed ( See 406). 



% 402. Wiring, Rheostat and Ammeter for a Projection Mi- 

 croscope. Every person who uses a projection apparatus should 

 know the simple but fundamental principles involved in its proper 

 installation and running. Fig. 206 illustrates the arrangement of all 

 the parts. The arrows indicate the direction of the current and the 

 plus sign indicates the positive and the minus sign the negative 

 terminal. It will be seen that all the parts, z". e. the resistance or 

 rheostat (3) the ammeter (4) the lamp (5 or 6), are placed in series 

 along one wire, and there is no connection with the other main wire 

 (2) until the current leaves the lamp. If at any other place a connec- 



