FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
15 L 
where the semi-absolute volt is taken as the potential difference between the terminals 
of an international ohm (Britain, America, and Germany) when it is traversed by a 
current of one absolute ampere. 
The writer desires as a result of the investigation recorded in this paper to add 
to the above list the value P01831 semi-absolute volts at 20° C., which has been 
determined with the aid of a Weber electrodynamometer in the possession of McGill 
University, Montreal. 
This instrument was purchased by the University as a part of the general equipment 
of the Macdonald Physics Building about 1893. It was carefully made by N alder, 
and was an exact copy of the dynamometer constructed by the Electrical Committee 
of the British Association, and described by Maxwell.* The instrument was set up by 
Mr. B. 0. King in 1895 under the direction of Prof. H. L. Callendar. The work 
by King has been fully described by Callendar t who made some important changes 
in the design of the instrument. Preliminary observations were obtained by King for 
the electromotive force of the old Board of Trade form of crystal Clark cell, and it was 
intended that more accurate work should be obtained later. This work was not, 
however, carried out, and the dynamometer remained in disuse for over ten years. It 
was suggested by Prof. Barnes that the writer should complete this investigation, 
and through the generosity of Mr. R. O. King, a Fellowship was provided, which 
enabled him to devote a year almost without interruption to the work. 
Several sources of error which appeared in the early use of electrodynamometers 
have been eliminated, and it is considered that an absolute accuracy closer than 1 part 
in 40,000 has been obtained in this investigation. Measurements can be repeated with 
the instrument to a much higher order of accuracy, but it was estimated that there 
might possibly be a constant error of 2 or 3 parts in 100,000. 
The deflection measurements, the determination of the ratio of the radii of the two 
sets of coils, and the evaluation of the controlling couple formed the main parts of the 
investigation. For the convenience of comparison all the observations on each factor 
are grouped together, and the final calculation for the mean electromotive force of the 
standard cell is calculated from the mean values for the various factors under known 
conditions. An illustration of the dynamometer is shown in Plate 1. Descriptive 
details of the various parts of the instrument appear in the respective sections of the 
paper. 
II. The Theory of the Weber Electrodynamometer. 
(a) The Expression for the Magnetic Couple Acting on the Suspended System when 
Currents are Flowing in the Coils. 
If W is the mutual potential energy of the two systems when currents i x and i 2 
flow through the fixed and suspended coils respectively, and if <p is equal to — 6, 
* Maxwell, ‘ Electricity and Magnetism’ (third edition), vol. 2, p. 367. 
t Callendar, ‘Phil. Trans.,’ A, vol. 199, p. 55, 1902. 
