132 Report of the Department of Bacteriology of the 



been portable; while if they had been broader and lower (15 inches 

 high, possibly) with one shelf instead of two, the temperature vari- 

 ation between top and botton could probably have been disregarded. 



The details of construction are similar to those of the refrigerator 

 except for the omission of pine lining and of the accessories for 

 supporting the ice. 



The walls separating the individual incubators consist solely of 

 four-inch cork-board, cemented together, the horizontal partitions 

 having, on their under surface, two thin bars of steel for support. 

 The construction is further strengthened by a steel rod passing 

 through the cork-board from the middle of the front to the middle 

 of the back, serving to prevent the front and back from spreading. 

 The inner surfaces of the cork are covered with the Nonpareil finish- 

 ing cement, except on the doors, where zinc is used; and both cement 

 and zinc are painted white. In each chamber there is a cherry 

 framework around the walls to support shelves and thermostat. 

 The two shelves are built of slats of half-inch seasoned cherry and 

 rest on metal pegs so that they can be adjusted to any height. 

 A fender about an inch in front of the thermostat, which stands at 

 the middle of the back wall, protects it from injury. 



HEATING SYSTEM. 



The arrangement of the circuits in the incubators is shown 

 diagrammatically in Fig. 5. In each incubator there are but two 

 essential pieces of apparatus, the thermostat T, and the resistance 

 wire R. The four incubators are connected with the main circuit 

 through the fuses FF, and are in parallel, just as are ordinary in- 

 candescent lights. Within the chambers, however, the thermostats 

 and heaters are in series. 



The thermostat is the most important part of the heating system. 

 The four thermostats were designed and made by Prof. C. H. Tower 

 of the electrical engineering department at Cornell University. 

 They are wholly of metal, with rigid bases, and can be firmly secured 

 in any desired position. The sensitive part of each is a strip of 

 copper and iron about twelve inches long. As this type of thermo- 

 stat interrupts the current when over-heated and can be wired into 

 the main circuit, power is used only when it is needed for the pro- 

 duction of heat. This results in economy. The initial cost, more- 

 over, of all four thermostats was only $12. 



The heaters are of Nichrome wire 4 No. 32, having a diameter of 

 0.008 inch. In each chamber are two lengths of wire, each about 

 20 feet long, shown in Fig. 5 by the zig-zag lines RR. In the in- 

 cubators, however, the turns of wire are in a horizontal instead of 

 a vertical plane, and each length is supported beneath a shelf. As 

 shown in Fig. 5, the two lengths of wire are not arranged the same 



4 Obtained of Driver-Harris Wire Co. of Harrison, N. J. 



