THE CALORIMETER SYSTE^I AND MEASUREMENT OF HEAT. 141 



junction system and two with wires extending to the galvanometer. 

 The special feature of this switch is the device for closing the circuit. 

 The two copper links, each of which is in fact composed of several 

 strands of No. 16 copper wire, are fastened to a square of hard rubber 

 on the end of a steel rod. A cross-section of a unit connection on this 

 switch is shown in figure 39. 



A hole drilled in a block of oak serves as the mercury cup. At the 

 bottom of each cup there is an iron or steel screw which extends through 

 the oaken block to a brass nut on the under side. The wires leading 

 to the galvanometer and thermal junction circuit, respectively, are sol- 

 dered to the brass nuts. The holes in the oaken block used for mercury 

 cups are shown at the left in figure 40. The wood was boiled for sev- 

 eral hours in paraffin and a thick coating of paraffin covers all of it. 

 The crater-like appearance of each hole is due to the deposit of par- 

 affin about the rim. 



The cover of the switch, which is not so thick as the base, is made 

 of mahogany. The steel rod to which the square of hard rubber with 

 the copper links is attached passes through a metal bushing set in the 

 cover, and the two copper links are held suspended over the mercury 

 cups by a spring coiled around the steel shaft. On the upper end of 

 the steel shaft a hard-rubber button is attached. A steel guide wire 

 driven into the cover and passing through a hole in the square of hard 

 rubber insures the copper links entering the mercury cups in the proper 

 position when the key is pressed. The manipulation is not unlike that 

 of depressing the key of a typewriter, save that the key is held down 

 for several seconds. The details of the under side of the cover are 

 shown in the right-hand portion of figure 40. 



The whole switch, with the cover in place, is shown in figure 41 . In 

 figure 37 the switch may be seen in position on the observer's table. 

 Some idea of the intricacy of the wiring is obtained from the view given 

 in figure 42, which shows the under side of the switch. 



The details of the electrical connections between the switch and the 

 different parts of the calorimeter with which it is concerned are illus- 

 trated in figure 43. 



Here it is seen that a number of connections other than those having 

 to do with the thermal junction system are included in this switch. To 

 distinguish the different thermal junction systems, we have designated 

 those belonging to the system between the two metal walls of the cham- 

 ber as No. i ; those in the air current as No. 2, and those in the inner 

 wooden walls as No. 3. The different sections of circuits Nos. i and 

 3 are further subdivided into top (T), upper zone (U), lower zone (I,), 

 and bottom (B). Consequently the circle on figure 43, in which the 



