ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 137 



however, will vary as the square of the current ; and, since treblingr the 

 current produces so small a change, we may infer that the total eflPect is 

 itself small. 



Another coil gave the following results : — 



indicating a change in the measured I'esistance of •0042 ohm on 100 

 ohms. 



It is clear, thei'efore, that the effect of heating is small, though appre- 

 ciable when currents approaching •IS ampere are used. 



APPENDIX III. 



On Standards of Loiv Electrical Resistance. By J. Viriamu Jones, 

 Principal and Professor of Physics in the University College, Cardiff. 



The preparation of standards of low electrical resistance of from 

 •001 to '0001 ohm seems to be a matter of some importance at the 

 present time. These standards are already in request among engineers, 

 and it becomes of interest to consider how they may best be measured to 

 a percentage accui*acy comparable with that with which the standard ohm 

 is known. 



Such standai'ds of low resistance may be derived by potentiometer 

 methods from the standard ohm by a series of downward steps. But this 

 is from one point of view roundabout. The method of measuring the 

 ohm that seems in all its details most accurate is that of Lorenz. In 

 this method the ohm itself is derived from the measurement of a small 

 resistance. It is simply going up and down again to prepare from the 

 ohm so derived the required small resistance standards, and it is more 

 direct and more accurate to measure the latter directly in absolute 

 measure. 



' In Lorenz's method a metallic disc is made to rotate in the mean 

 plane of a coaxial standard coil. Wires touching the centre and circumfer- 

 ence of the disc are led to the ends of the resistance to be measured, 

 and the same current is passed through this resistance and the standard 

 coil. The connections being rightly made, we may by varying either the 

 rate of rotation of the disc or the resistance measured so arrange matters 

 as to have no change of current in the circuit of the disc and wires joining 

 it to the ends of the resistance, when the direction of the current through 

 the resistance and the standard coil is changed. When this arrangement 

 is effected there is a balance between the electromotive force, due to the 

 motion of the disc in the magnetic field of the current in the standard 

 coil, and the difference of potential at the ends of the resistance, due to 



