24 L. B. Fletcher — JL Determination of the B. A. Unit 



jacket was provided with a hollow cover, also filled with water, 

 and its inner surface and the outer surface of the calorimeter 

 were nickel-plated and polished. Thus the calorimeter was 

 nearly surrounded by an envelope of fairly constant temper- 

 ature, the thermometer, stirrer-thread and connecting wires 

 passing through openings in the jacket. 



From the mercury cups in which the electrodes of the calori- 

 meter dipped, the wires of the main circuit ran to the battery 

 and galvanometer. These wires were 2 - 5 mm in diameter, cotton- 

 covered, carefully paraffined and twisted together to eliminate 

 direct action on the needle. The battery consisted usually of 

 24 one-gallon bichromate cells arranged 4 in series and 6 abreast, 

 and gave a very steady current. In one experiment only 20 

 cells were used, 4 in series and 5 abreast. The galvanometer 

 coil for the main current was a single turn of stout wire laid in 

 a groove on a wooden circle of about 80 cm diameter. A sine 

 galvanometer was so placed that its needle was in the axis of 

 the single wire coil and about 'l cm distant from its plane. 

 This excentricity was rendered necessary by the length of the 

 suspending fibre. The coil of the sine galvanometer was con- 

 nected with the calorimeter electrodes by a second circuit, in 

 which a resistance coil of 30,000 ohms was included. The 

 wires of this circuit were kept apart, as the current was too 

 small to exert an appreciable direct action, and as great irregu- 

 larity in some preliminary experiments in which the wires were 

 twisted together was finally traced to leakage, although the 

 wires had a double covering of silk. Both circuits were pro- 

 vided with commutators. The sine galvanometer had a hori- 

 zontal bar parallel to the axis of the coil. To one end of this 

 was attached a telescope, beneath which was a short scale 

 which was seen by reflection in the mirror of the needle, and 

 allowed a very accurate setting to be made without bringing 

 the needle to rest. The needle consisted of two thin strips of 

 steel l - 2 cm in length separated by a piece of wood '6 cm in thick- 

 ness. The circle of the galvanometer was graduated to half- 

 degrees and read by verniers to one minute. 



The needle was acted upon by both currents simultaneouly, 

 and by means of the commutators the actions were caused to 

 be in the same and in opposite directions alternately. The 

 current through the sine galvanometer is c' in the formula 



cc'H't. 

 J= — 7 — ' The current through the coil on the w T ooden circle 



is c-f-c', and was assumed equal to c as c' was less than - 00007c. 

 Let G denote the constant of the fixed coil, G' that of the sine 

 galvanometer, H the horizontal magnetic force, d and 6' the 

 deflections when the actions are in the same and in opposite 

 directions respectively. Then 



