THE CULTIVATION OF MICRO-ORGANISMS 127 



end travels away from the set-screw at h; when it cools, it moves 

 toward the set-screw. Rogers also recommends a regulator 

 made of invar and brass instead of hard rubber and brass. Where 

 invar is used instead of the hard rubber the dimensions for the 

 two metals are the same as those given for the brass strip in the 

 hard-rubber-brass regulator just described. As is evident from 

 the description, the circuit controlling the magnet is closed when- 

 ever the free end of the regulator comes in contact with the set 

 screw at h. When this circuit is closed the magnet attracts the 

 armature, and the heating circuit is closed by the contact formed 

 at c between the armature and the set-screw. In the diagram 

 this point of contact is put to one side for the sake of clearness, 

 but as a matter of fact in the instrument in use, the set-screw is 

 above and between the ends of the horse-shoe magnet, and comes 

 in contact with the armature which is extended upward in the 

 shape of a tongue. From the description just given it will be 

 noted that the thermoregulator does not control the heating 

 directly, but indirectly through the electro-magnet. 



Certain precautions have been found necessary in practice 

 in order to obtain satisfactory results with this incubator. The 

 set-screw against which the armature strikes at c should be so 

 set that the armature does not come in contact with the magnet. 

 In the apparatus described above there is a space of about M inch 

 between the armature and the magnet when contact takes place 

 between the set-screw and the armature. If the set-screw does 

 not project far enough to prevent the armature from coming in 

 contact with the magnet, the arm.ature may adhere to the magnet 

 even after the current is broken at b, and when this is the case of 

 course the lamp remains lighted, and the temperature may run 

 up too high. This sticking of the armature to the magnet is 

 said to be due to the residual magnetism left in the core of the 

 magnet. When the current passing through the magnet is 

 broken by the excursion of the end of the thermoregulator away 

 from the set-screw at h, the armature is pulled away from the 

 magnet by a coiled spring. Another important precaution is 



