XXXVlll INTRODUCTION. 



for consideration for fundamental definition, both being units of quantities 

 more fundamental in electrical theory than resistance." 



For all practical purposes the "international" and the "practical" or "abso- 

 lute" units are the same. Experimental determination of the ratios of the corres- 

 ponding units in the two systems have been made and the mean results are 

 given in Table 382. These ratios represent the accuracy with which it was possible 

 to fix the values of the international ohm and ampere at the time they were 

 defined (London Conference of 1908). It is unhkely that the definitions of the 

 international units will be changed in the near future to make the agreement 

 any closer. An act approved July 12, 1894, makes the International units as 

 above defined the legal units in the United States of America. 



THE STANDARDS OF THE INTERNATIONAL ELECTRICAL 



UNITS. 



RESISTANCE 



Resistance. — The definition of the international ohm adopted by the London 

 Conference in 1908 is accepted practically everywhere. 



Mercury Standards. — Mercury standards conforming to the definition were 

 constructed in England, France, Germany, Japan, Russia and the United States. 

 Their mean resistances agree to about two parts in 100,000. To attain this 

 accuracy, elaborate and painstaking experiments were necessary. Tubes are 

 never quite uniform in cross- section; the accurate measurement of the mass of 

 mercury filling the tube is difl&cult, partly because of a surface film on the walls 

 of the tube; the greatest refinements are necessary in determining the length of 

 the tube. In the electrical comparison of the resistance with wire standards, 

 the largest source of error is in the filling of the tube. These and other sources 

 of error necessitated a certain uniformity in the setting up of mercury standards 

 and at the London Conference the following specifications were drawn up: 



SPECIFICATION RELATING TO MERCURY STANDARDS OF RESISTANCE. 



The glass tubes used for mercury standards of resistance must be made of a glass such that 

 the dimensions may remain as constant as possible. The tubes must be well annealed and straight. 

 The bore must be as nearly as possible uniform and circular, and the area of cross-section of the 

 bore must be approximately one square millimeter. The mercury must have a resistance of 

 approximately one ohm. 



Each of the tubes must be accurately calibrated. The correction to be applied to allow for 

 the area of the cross-section of the bore not being exactly the same at all parts of the tube must 

 not exceed 5 parts in 10,000. 



The mercury filling the tube must be considered as bounded by plane surfaces placed in 

 contact with the ends of the tube. 



The length of the axis of the tube, the mass of mercury the tube contains, and the electrical 

 resistance of the mercury are to be determined at a temperature as near to 0° C as possible. 

 The measurements are to be corrected to 0° C. 



For the purpose of the electrical measurements, end vessels carrying connections for the 

 current and potential terminals are to be fitted on to the tube. These end vessels are to be 

 spherical in shape (of a diameter of approximately four centimeters) and should have cylindrical 

 pieces attached to make connections with the tubes. The outside edge of each end of the tube 



