88 



MANUAL OF BACTERIOLOGY. 



which is inserted in the heating circuit in such a way that 

 when the armature is attracted toward the magnet the circuit 

 is completed and the lamp is consequently lighted. The part 

 of the current, a, supplying the magnet first passes through 

 a small lamp and through two resistance coils so as to reduce 

 the pressure. It then passes through the magnet, and is con- 

 tinued on to the set-screw, b, which is so placed that when the 

 thermoregulator comes in contact with it the current is com- 

 plete. The regulator consists of a strip of hard rubber and a 



-Kara/ Rubber: 

 Thermo rea/u 'later 



FIG. 27. 



'strip of brass riveted together. One end is fixed, while the 

 other is free, and when it is heated it tends to bend toward 

 the metal side, when it cools it bends toward the rubber. 

 The brass strip is 15 inches long, J inch thick, and J inch wide; 

 the rubber strip is \ inch thick, \ inch wide, and a little less 

 than 15 inches long. In the diagram the end is fixed at d 

 and is free at b. When it is heated, the free end travels away 

 from the set-screw at b; when it cools, if moves toward the set- 

 screw. Rogers also recommends a regulator made of invar and 



