212 ROSA, DORSEY AND MILLER: INTERNATIONAL AMPERE 



only of ratios of the dimensions of the electrical circuit. To 

 measure the current I, the force F must be measured and K must 

 be determined. As to the method of measuring F and deter- 

 mining K, the various types of instruments differ. 

 The force or torque may be balanced by 



(a) The elastic deformation of a wire or fiber. 



(b) The gravitational attraction of the earth. 



Then the torque or force must in turn be evaluated in absolute 

 units. 



K may be determined in three ways 



(c) by measurements of the separate dimensions of the instrument. 



(d) 1 y comparisons of the instrument with some other instrument, 

 the dimensions of which can better be measured. 



(e) by measurements of the ratios of the dimensions which enter 

 into K. 



The accuracy attainable with the instrument depends upon the 

 possible accuracy of these determinations. 



The method (a) requires a direct determination of the force of resti- 

 tution of the deformed body (generally a fiber) as it was used in the 

 electrical measurements. The elastic after-effects shown by all mate- 

 rial bodies introduce formidable difficulties. 



The method (b) requires the measurement of a force by weighing, 

 and the absolute value of the acceleration of gravity. The first is capa- 

 ble of extreme precision. An accurate determination of the second is 

 very difficult, but results obtained with this method at different times 

 and places can be very accurately compared by relative determinations 

 of the acceleration of gravity and may be corrected at any time when 

 the absolute value is determined to a higher degree of accuracy. 



The determination of the constant K by the method given in (c) 

 requires that the geometrical form of the apparatus be simple and the 

 dimensions relatively large. Thus, the use of single layer coils is neces- 

 sitated in general. The forces of attraction are usually small and can, 

 therefore, be measured accurately only with difficulty. Furthermore, 

 the constant is determined once for all and must be assumed to remain 

 constant. 



The method (d) allows the use of multiple layer coils and some other 

 features which tend to give a high sensibility in non-absolute instru- 



