ON STANDARDS OF ELECTRICAL RESISTANCE. 513 



due to the distance between the fixed coils. In my instrument, in which 

 this distance is 1 inch, the diameter of the coils being 12 inches, and their 

 interior core 4 inches, this correction was proved by experiment to be -1185. 



There was, however, considerable difficulty in obtaining an exact measure 

 of the distance between the fixed coils, and I therefore judged that the mea- 

 sure of the currents used in the experiments would be most accurately 

 obtained by means of a tangent galvanometer, the above described current- 

 meter being employed to determine the horizontal intensity. 



This determination was efi'ected as follows : — Many careful observations 

 of the horizontal intensity by an improved method on Gauss and Weber's 

 system were made alternatively vrith observations of the deflections of a 

 tangent galvanometer and the weighings of the current-meter when the 

 same currents travei'sed both instruments in succession. Then calling the ho- 

 rizontal intensity H, the angle of deflection d, and the weighing w, there was 



obtained a constant c = — 7^;^= -17676. Hence with these instruments 



yw 



H= 



•17676 Vm' 



tany 



The experiments for the determinations of horizontal intensity by the use 

 of this formula coidd be efi'ected in a few minutes, and did not require an 

 alteration in the disposition of any part of the apparatus. It was satisfac- 

 tory to find that, although the presence of masses of iron at only a few j'ards 

 distance made the field in which I worked considerably more intense than 

 that due to the latitude, and although I worked at diflferent times of the day, 

 the highest intensity, oiit of upwards of seventy observations distributed over 

 a year, was 3-6853, and the lowest 3-6607, indicating a much greater degree 

 of constancy than might have been expected. 



The galvanometer above mentioned was that employed in the thermal 

 experiments. It had a single circle of ^-inch copper wire, the diameter 

 of which, being measured in many places by a standard rule, gave a radius 

 of -62723 of a foot. The needle was half an inch long, and furnished with 

 a glass pointer traversing a divided circle of 6 inches diameter. In the experi- 

 ments the deflections were not far from 26° 34', the angle at which the influ- 

 ence of the length of the needle within certain hmits is inappreciable. It was 

 easy by a magnifler, arranged so as to avoid parallax, to read to one minute. 

 The torsion of the fibre gave only 3'-5 for an entire twist. The trifling cor- 

 rection thus required is applied to the recorded observations of deflection. 



The calorimeter first used was a copper vessel upwards of a gallon in 

 capacity, filled with distilled water. It had a conical lid, attached by screws, 

 in which were two tubulures, one for the introduction of a copper stirrer, 

 the other for the thermometer, around the immersed stem of wliich a wire of 

 platinum silver, having a resistance nearly equal to that of the Association 

 unit, was coiled. 



The resistance of the wire was found by comparing it with the Association 



n /■n n \ 



unit, sent me by the Committee, using Ohm's formula, .r=-?( -a — ^ ' ], where 



C3, C^, and Cj are the tangents of defiection with the battery and connexions 

 only with these and the unit and with the coil respectively. This, though 

 by no means so delicate a method as that of the Wheatstone balance im- 

 proved by Thomson, was able to give a final result certainly accurate to the 

 two-thousandth part. The results for the resistance of the coil in the first 

 series of experiments are as follow. They were obtained before and after 

 1867. 2 M 



