TRANSACTIONS OF SECTION A. 



327 



Department of General Physics. 



The following Papers were read : — 



1. The Absolute Measurement of Current at the Bureau of Standards. 



By Dr. N. E. Dorsey. 



A balance of the type used by Lord Rayleigh was employed. The coils of 

 square cross-section are wound bifilarly of enamel-insulated wire upon brass 

 forms. A novel feature of the fixed coils is the provision in the forms and 

 back of the windings of a channel through which water can be pumped so as to 

 maintain the coils at a constant temperature. During the weighings the moving 

 coil was surrounded by a double-walled jacket maintained at a constant tem- 

 perature by water-circulation. 



Three pairs of fixed coils (two 50 cm. in diameter and of thirty-six layers 

 each of eighteen double turns, and one of 40 cm. diameter and twenty-eight 

 layers of fourteen double turns each) and two moving coils (one 25 cm. in 

 diameter and of twelve layers of six double turns, the other 20 cm. in diameter 

 and of fourteen layers of seven double turns) were used in the final work. In 

 the earlier work two other moving coils not well suited for absolute results 

 were used. 



All portions of the instrument have been tested by means of a very sensitive 

 astatic magnetometer and have been found to be good. The insulation of the 

 coils has been excellent throughout the work. 



After winding, the coils were carefully sealed with paraffined cloth and 

 paraffin, and covered by a layer of softer wax. The windings are thus well pro- 

 tected from changes in the atmospheric humidity. The distance between the 

 two fixed coils was always such that the sum of the forces which they exerted 

 upon the moving coil was a maximum. Under this condition the maximum 

 force for a given current depends solely upon the ratios of the diameter of the 

 moving coil to those of the fixed coils. Consequently the actual distance 

 between the fixed coils need not be measured. This is a great advantage. The 

 ratios of the diameters of the coils were obtained by a modification of the elec- 

 trical method used by Lord Rayleigh. The settings of the coils and of the 

 needle were adjusted by electrical methods; and the variations in the diameters 

 produced by variations in the temperature and in the load carried were care- 

 fully studied experimentally. It has been found to be practicable to attain 

 such an accuracy in the measurement of the ratio of the galvanometer constants 

 of such coils that the mean variation from the mean of ten or more measure- 

 ments shall amount to but two in a million. The correct adjustment of the 

 coils was determined electrically, and an electrical method was devised for 

 detecting and correcting for any slight error that might exist in the spacing or 

 in the coaxiality of the fixed coils. The horizontality of the coils was tested by 

 means, of delicate levels. 



While the earlier results are more erratic than the later ones, they give the 

 same mean value. However, the conditions under which they were obtained 

 were much less satisfactory than those secured later, and consequently only the 

 later values have been considered in obtaining the final conclusion. These 

 observations give the following values for the electromotive force of the mean 

 Weston normal cell (as defined at the Washington Conference) at 20° C. in 

 terras of the international ohm and the Bureau of Standards' balances : — 



