CHAPTER X. 

 ELECTRICAL MEASUREMENTS. 



148. Absolute measurements and international standards. 

 The measurement of an electrical quantity in terms of the me- 

 chanical units of length, mass and time directly is called "abso- 

 lute" electrical measurement. For example, the measurement 

 of current by the Weber electro-dynamometer as explained in 

 Art. 59, is an " absolute " measurement. Absolute electrical 

 measurement requires, in most cases, elaborate apparatus, and, 

 unless extreme precautions are taken, is subject to considerable 

 error. In consequence of this fact a standard of resistance and 

 the electrochemical equivalent of silver have been measured " ab- 

 solutely " with extreme care and adopted as international stand- 

 ards,* and all ordinary electrical measurements consist in the 

 comparison of the quantity to be measured with these standards. 



MEASUREMENT OF CURRENT. 



149. Measurement of current by electrolysis. The electro- 

 chemical equivalent of a metal having been determined once for 

 all, the strength of any current may be easily and accurately 



*Tbe international standard ampere is defined in Art. 3, and the method by which 

 it was determined is described in Art. 59. The international standard ohm is defined 

 in Art. 52, and the method by which it was determined is described in Art. 152. It 

 is likely that the electromotive force of the Clark standard cell (see Art. 159) will be 

 adopted as an international standard at the next International Electrical Congress. 

 In fact, all practical electrical measurements are now based upon the standard cell 

 and a standard ohm. The use of the silver voltameter is very tedious and the results 

 obtained are less reliable than those which may be obtained with great ease by the 

 use of a standard ohm and a standard cell. 



An historical sketch of the international units by Frank A. Wolff is to be found in 

 the Bulletin of the United States Bureau of Standards, Vol. I, pages 39-76. The 

 Acts of Congress establishing the legal electrical units for the United States are given 

 on pages 61-65. 



276 



