THE REGULATION OF TEMPERATURE. 59 



a battery of one cell; (2) a thermostat; (3) two relays; (4) three lamps 

 (I) and three condensers (c) which span the spark gaps of the ther- 

 mostat and the two relays; (5) two battery circuits which operate the 

 relays and a third one of 120 volts which passes through the stove. 



1. The Battery. 



For charging purposes, all the cells in use (about twenty in number) 

 are placed in series upon a single circuit in which a lamp of appro- 

 priate resistance is also inserted. The charging is continuous. The 

 cells themselves, though in series on a single circuit for charging, are 

 distributed, in numbers corresponding to the amount of work to be done, 

 at points conveniently near to the various baths. A single cell suffices 

 to operate the system at any point, hence each cell in a local battery 

 consisting of more than one is at work only a part of the time, i. e., 

 every third day, if the local battery consists of three cells. 



2. The Thermostat. 



No single element in a system of regulation is of greater importance 

 than the thermostat. Its efficiency depends upon a considerable 

 number of conditions, some of which are worthy of more than a passing 

 mention. That the mercury must be of exceptional purity, and that 

 its volume must be so related to the diameter of the capillary as to 

 secure a large movement of the meniscus for a small change of tem- 

 perature, are facts too obvious to require discussion. The feature to 

 which too little attention is usually given is the mechanism for adjust- 

 ing the contact point. This should be located directly over the center, 

 i. e., the highest part of the meniscus, and the mechanism should be 

 such that it can never take any other position with reference to the 

 surface of the meniscus. The best form of thermostat which we have 

 in use is shown in Figure 30. The platinum rod (a) is finished to a 

 smooth point at the lower end, and just above the latter is a guide 

 (b) of glass, which is designed to keep the point near the center of the 

 tube, and therefore nearly over the highest part of the meniscus. At 

 the upper end, the platinum rod (a) is firmly set in the threaded brass 

 rod (c). The adjustment is made by means of the nut (e, e), which 

 is so nicely fitted into its framework that it can move in a horizontal- 

 circular direction only. The dotted circle indicates the apertures 

 through which the adjusting nut (e, e) is grasped between the thumb 

 and forefinger when the contact-point is to be lowered or raised. The 

 guide (Jo), which must fit the tube rather loosely, does not suffice to 

 compel the contact-point to keep exactly its proper position with 

 reference to the meniscus. The rod (a) is never absolutely straight, 

 hence the point, if the rod is allowed to turn, will describe a circle 

 over the meniscus. For this reason, having once correctly adjusted 

 the point, its motion must be limited to the vertical direction; in other 

 words, the threaded rod (c) must not be allowed to turn with the nut 



