ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 45 
second ; (4) the watt, the unit of power which has the value 10,000,000 
in terms of the centimetre, the gramme, and the second. 
II. As a system of units representing the above, and sufficiently 
near to them to be adopted for the purpose of electrical measurements 
and as a basis for legislation, the Conference recommends the adoption 
of the international ohm, the international ampere, and the inter- 
national volt defined according to the following definitions : — 
III. The ohm is the first primary unit. 
IV. The international ohm is defined as the resistance of a specified 
column of mercury. 
V. The international ohm is the resistance offered to an unvarying 
electric current by a column of mercury at the temperature of melting 
ice, 14°4521 grammes in mass, of a constant cross-sectional area and 
of a length of 106°300 centimetres. 
To determine the resistance of a column of mercury in terms of 
the international ohm, the procedure to be followed shall be that set 
out in Specification I. attached to these Resolutions. 
VI. The ampere is the second primary unit. 
VII. The international ampere is the unvarying electric current 
which, when passed through a solution of nitrate of silver in water, in 
accordance with Specification II. attached to these Resolutions, deposits 
silver at the rate of 0°00111800 of a gramme per second. 
VIII. The international volt is the electrical pressure which, when 
steadily applied to a conductor whose resistance is one international 
ohm, will produce a current of one international ampere. 
IX. The international watt is the energy expended per second by 
an unvarying electric current of one international ampere under an 
electric pressure of one international volt. 
Specirication I. 
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 millimetre. 
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 deter- 
mined at a temperature as near to 0° C. as possible. The measure- 
ments are to be corrected to 0° C. 
_ For the purpose of the electrical measurements, end vessels carry- 
ing 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 fon centimetres) and should haye cylindrical 
