CARBON DIOXIDE AT DIFFERENT TEMPERATURES. 207 



Joules/cal. at 20 C., obtained by BARNES* for the mechanical equivalent of heat, 

 corresponds to the international ohm, and to the value 1'4334 volts for the E.M.F. of 

 the Clark cell at 15 C., this value for the Clark cell being obtained by direct 

 measurement in terms of the international ohm and the ampere (10" 1 C.G.S. unit).f 

 Thus, by using the above value for the cadmium cell and taking the international ohm 

 as the unit of resistance, the specific heats are expressed in terms of the calorie at 

 20 C., which is assumed to be equal to 4 '180 Joules. 



(8) The Potentiometer. The potentiometer was of the form designed by 

 Prof. CALLENDAR. The principle involved is similar to that involved in the Thomson- 

 Varley slide, but the instrument has four dials instead of two. The second dial could 

 be made to form a shunt across any adjacent pair of coils of the first ; the third dial 

 was related in a similar way to the second, and the fourth to the third. The 

 resistances of the coils in the different dials were so adjusted that the potentiometer 

 read directly in a manner somewhat similar to the Thomson- Varley instrument ; the 

 thousands were read off on the first dial, the hundreds on the second, the tens on the 

 third, and the units on the fourth. A full account of the details of the calibration 

 of the instrument, together with the tables of observations, have been preserved, and 

 since it can be shown from them that by no conceivable arrangement of the sliders 

 can an error of as much as 1 part in 10,000 arise in the actual potentiometer 

 readings through neglect of the calibration corrections, no corrections have been 

 applied. 



The current for the potentiometer was supplied by two freshly made up Leclanche 

 cells. The galvanometer used was of the Thomson pattern ; it had a resistance of 

 7000 ohms, and the sensitiveness in the main experiments was such that a change of 

 unit amount in lowest dial of the potentiometer produced an alteration of 1 divisions 

 in the deflection. 



(9) The Secondary Standard Resistance. The coil was wound on a wooden frame 

 from about 50 cm. of No. 22 manganin wire, and it was placed in a double-walled 

 vessel containing oil. Water from the tap passed through the space between the 

 double walls, and the temperature could be measured by a thermometer passing 

 through the lid and dipping into the oil. The ends of the coil were soldered to two 

 thick copper U -pieces, the ends of which were connected to four brass terminals, two 

 for the current leads and two for the potential leads. The secondary standard was 

 compared with a primary standard 1 ohm coil by passing the same current through 

 each, and comparing the potential differences between the ends of the coils on the 

 potentiometer ; thus, all question as to the resistances of the leads was avoided. The 

 comparison having been made, the current of water passing through the jacket of the 

 coil was stopped ; the electric current was caused to flow through the secondary 



* ' Phil. Trans.,' A, 314 (1902). 



t ' Phil. Trans.,' A, 313, p. 86 (1902). 



