INTRODUCTION. XXxix 



is to be coincident with the inner surface of the corresponding end vessel. The leads which make 

 contact with the mercury are to be of thin platinum wire fused into glass. The point of entry 

 of the current lead and the end of the tube are to be at opposite ends of a diameter of the bulb; 

 the potential lead is to be midway between these two points. All the leads must be so thin 

 that no error in the resistance is introduced through conduction Of heat to the mercury. The 

 filling of the tube with mercury for the purpose of the resistance measurements must be carried 

 out under the same conditions as the filling for the determination of the mass. 



The resistance which has to be added to the resistance of the tube to allow for the effect ot 

 the end vessels is to be calculated by the formula 



A = -^ — ( - + 1 ) ohm, 

 io637r \ri rz/ 



where ri and rz are the radii in millimeters of the end sections of the bore of the tube. 



The mean of the calculated resistances of at least five tubes shall be taken to determine the 

 value of the unit of resistance. 



For the purpose of the comparison of resistances with a mercury tube the measurements 

 shall be made with at least three separate fillings of the tube. 



Secondary Standards. — Secondary standards, derived from the mercury 

 standards and used to give values to working standards, are certain coils of 

 manganin wire kept in the national laboratories. Their resistances are adjusted 

 to correspond to the unit or its decimal multiples or submultiples. The values 

 assigned to these coils are checked from time to time with the similar coils of 

 the other countries. The value now in use is based on the comparison made 

 at the U. S. Bureau of Standards in 1910 and may be called the "1910 ohm." 

 Later measurements on various mercury standards checked the value then used 

 within 2 parts in 100,000. Thus the basis of resistance measurement is main- 

 tained not by the mercury standards of a single laboratory, but by all the mer- 

 cury standards of the various national laboratories; it is furthermore the same 

 in all countries, except for very slight outstanding discrepancies due to the 

 errors of measurement and variations of the standards with time. 



Resistance Standards in Practice. — In ordinary measurements, working 

 standards of resistance are usually coils of manganin wire (approximately 84 

 per cent Cu + 12 per cent Mn + 4 per cent Ni). They are generally used in oil 

 which carries away the heat developed by the current and facihtates regulation 

 and measurement of the temperature. The best type is inclosed in a sealed case 

 for protection against atmospheric humidity. Varying humidity changes the 

 resistance of open coils often to several parts in 10,000 higher in summer than 

 in winter. While sealed i ohm and o.i ohm coils may remain constant to about 

 I part in 100,000. 



Absolute Ohm. — The absolute measurement of resistance involves the pre- 

 cise determination of a length and a time (usually an angular velocity) in a 

 medium of unit permeabihty. Since the dimensional formula of resistance in 

 the electromagnetic system is \iLfj./T2, such an absolute measurement gives R 

 not in cm/sec. but in cm x fx/sec. The definitions of the ohm, ampere and 

 volt by the 1908 London conference tacitly assume a permeability equal to 

 unity. The relation of the international ohm to the absolute ohm has been 

 measured in different ways involving revolving coil, revolving disk, and alter- 

 nate current methods. Probably the most accurate determination was made 



