JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. I, NOVEMBER 19, 1911. \,,. g. 



ELECTRICITY.— .4 determination of the international ampere 

 in absolute measure. E. B. Rosa, N. E. Dorset and J. M. 

 Miller, Bureau of Standards. 



The absolute unit of current in the electromagnetic system is 

 denned in terms of the magnetic field which it produces. Hence 

 we can measure current in absolute units by measuring the mag- 

 netic field due to the current, or by measuring the ratio between 

 this field and some other field which is measurable, as, for example, 

 the earth's magnetic field. The comparison may be made by 

 means of the tangent galvanometer, and is often called the tangent 

 galvanometer method. 



Or, we can calculate the force of attraction between circuits 

 carrying electrical currents, and determine the current by meas- 

 uring the forces of attraction experimentally; this is called the 

 electrodynamometer or current balance method. The potential 

 energy of two circuits relative to one another is equal to IJ 2 M 

 where Ii and It are the strengths of the currents flowing in the 

 two circuits and M is the flux of magnetic force through one cir- 

 cuit produced by a unit current in the other, that is, the mutual 

 inductance. The force or torque between the two circuits tend- 

 ing to produce a displacement is measured by the rate of change 

 of the potential energy. If the currents are constant, we have to 

 consider, therefore, only the rate of change of the mutual induc- 

 tance. If a particular displacement be represented by a chai 

 dx in the variable x, and the current is the same in the two cir- 

 cuits, the force F =f ' = KI 2 . in which .1/ has the dimensions 



ox 



of aline; hence — - = K is of zero dimensions, and is a function 



211 



