THE CALORIMETER. 17 



monly used in the construction of refrigerators. This is shown as E in fig. 7, 

 while the outer asbestos panels are shown as F. 



A detail of the construction of the walls, showing in addition the heating 

 and cooling devices, is given in fig. 12, in which the copper is shown held 

 firmly to the upright channel H by means of the bolt I, screwing into a 

 brass or copper disk K soldered to the copper wall. The bolt I serves the 

 purpose of holding the copper to the upright channel and likewise by means 

 of a washer under the head of the screw holds the zinc to the channel. In 

 order to hold the asbestos-lumber panel A with the hair-felt lining B a 

 threaded rod E is screwed into a tapped hole in the outer part of the upright 

 channel H. A small piece of brass or iron tubing, cut to the proper length, 

 is slipped over this rod and the asbestos lumber held in position by a hex- 

 agonal nut with washer on the threaded rod E. In this manner great rig- 

 idity of construction is secured, and we have two air-spaces corresponding to 

 the dead air-spaces indicated in fig. 7, the first between the copper and zinc 

 and the second between the zinc and hair-felt. 



PREVENTION OF RADIATION. 



As can be seen from these drawings the whole construction of the appa- 

 ratus is more or less of the refrigerator type, i. e., there is little opportunity 

 for radiation or conduction of heat. Such a construction could be multi- 

 plied a number of times, giving a greater number of insulating walls, and 

 perhaps reducing radiation to the minimum, but for extreme accuracy in 

 calorimetric investigations it is necessary to insure the absence of radiation, 

 and hence we have retained the ingenious device of Eosa, by which an 

 attempt is made arbitrarily to alter the temperature of the zinc wall so that 

 it always follows any fluctuations in the temperature of the copper wall. 

 To this end it is necessary to know first that there is a temperature differ- 

 ence between zinc and copper and, second, to have some method for con- 

 trolling the temperature of the zinc. Leaving for a moment the question 

 of measuring the temperature differences betwen zinc and copper, we can 

 consider here the methods for controlling the temperature of the zinc wall. 



If it is found necessary to warm the zinc wall, a current of electricity is 

 passed through the resistance wire W, fig. 12. This wire is maintained 

 approximately in the middle of the air-space between the zinc wall and hair- 

 felt by winding it around an ordinary porcelain insulator F, held in posi- 

 tion by a threaded rod screwed into a brass disk soldered to the zinc wall. 

 A nut on the end of the threaded rod holds the insulator in position. Much 

 difficulty was had in securing a resistance wire that would at the same time 

 furnish reasonably high resistance and would not crystallize or become 

 brittle and would not rust. At present the best results have been obtained 

 by using enameled manganin wire. The wire used is No. 28 American wire- 

 gage and has resistance of approximately 1.54 ohms per foot. The total 



