THE CULTIVATION OF BACTERIA. 89 



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 b. 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 descrip- 

 tion just given it will be noted that the thermoregulator does 

 not regulate 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 mag- 

 net. In the apparatus described above there is a space of ab- 

 out I 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 armature is apt 

 to stick 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 



* Recently the regulator has been connected directly with the current pass- 

 ing through the heating lamp without the intervention of the horse shoe mag- 

 net, connecting the wires e and / directly with d and b. 



