138 Report of the Department of Bacteriology of the 



The total cost of maintenance is thus seen to be about $40 per 

 year if based on the estimates of $5 per ton of ice and 10 cents per 

 kilowatthour of electric power. These estimates are purposely 

 high, and with the rates current here the cost is much less. 



EFFICIENCY OF APPARATUS. 



The efficiency of an incubator for bacteriological work is prin- 

 cipally a matter of accuracy in temperature control. The tempera- 

 ture regularity of this incubator may be seen at a glance from Fig. 7 



9 A.M. 



9 A.M. 



9 AM. 



9 A.M. 



9. AM. 



Fig. 7. — Thermograph Ee cords. 

 At 17.50°, 25°, 30°, 37° and 40° C. 



which contains temperature curves obtained by means of a ther- 

 mograph in the various chambers at 17.5°, 25°, 30°, 37° and 40°. 

 All the variations are within one degree Centigrade except the one 

 great irregularity in the 30° curve during the fifth day; and this 

 irregularity was the result of shutting off the power for two hours. 

 The most noticeable of all the other irregularities occur early in 

 the morning; and it is thought they must be due in some way to 

 the suddenly increased demand for electric power at the beginning 

 of Station hours. The low temperature curves are particularly 

 regular. This is especially interesting because of the difficulty 

 often experienced in obtaining efficient incubators for temperatures 

 below that of the room. 



There is a difference in temperature, fully as great as this, between 

 the top shelf and the floor of each incubator. This difference is 

 2° for chambers run at 30° or 37°, and 1° for chambers run at 18° 

 or 25°. With chambers as high as these this difference is inevitable; 

 but it can be allowed for by using the same shelf always for the 

 same work. 



In the refrigerator no attempt has been made to secure regularity 

 in temperature. It varies with the outside temperature and the 

 amount of ice within, and ranges between 7° and 10° C. 



