and Laboratory Methods. 



2333 



certain extent its expansive properties, so that it fails to release the lever and 

 cut off the gas. 



The departments of Botany and Zoology were already in possession of a 

 number of copper water-baths for heating parafifine and warm ovens of various 

 constructions, which it was desirable to obtain if they could be provided with a 

 suitable electric heating apparatus. With the aid of suggestions from Professor 

 Sabine of the department of Physics and the cooperation of constructors of elec- 

 trical apparatus, a plan was finally worked out which answers fairly well the 

 requirements of the ordinary parafifine water-baths. 



Fig. 1. — Paraffine water-bath and automatic electric heating apparatus. The 

 150-ohm telegraphic relay apparatus is mounted on the top of box contain- 

 ing a one-cell battery. Plug in place in the receptacle. At the right the 

 plug of another heating apparatus removed from receptacle and hung on a 

 hook. At the left a detached heating coil and insulated wire cable hanging 

 on the wall. A'., regulator ; T., thermometer ; W., wires of heating circuit ; 

 W ., wires of relay circuit. 



There were two chief problems to be solved : first, to ascertain the minimum 

 heating capacity of the heating coil necessary for each bath ; secondly, to devise 

 an automatic regulator to control the electric heating current. To reduce the 

 amount of heat lost, each bath was provided with a felt jacket, — covering all 

 parts except the top, — enclosed in a canvas cover. The felt was about half an 

 inch thick, and the canvas cover, after being slipped over the felt, was laced up 

 on one side, as is shown above the faucet in Fig. 1. The manufacturers of 

 the heating apparatus employed (the Simplex Electric Co. of Boston and Cam- 

 bridge) then determined empirically the proper resistance and length of coil 

 required to maintain a given temperature in each of the different forms and 

 sizes of baths. To insure uniform distribution of heat, the coil should make at 



