RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
87 
coil is of special construction; it is divided into two equal coils each of which can be 
compared with the standard coils. 
The coils were supported from mercury cups in a bath of well stirred paraffin oil 
maintained at a constant temperature of 20°'0 C. The stirring was produced by 
blowing dry air through the oil. A spiral toluene thermostat was used to control the 
temperature and the heating of the oil was produced by an electric current flowing 
through a resistance coil supported on a large frame at the bottom of the bath. 
These arrangements are those in common use in the Electrical Standards Department, 
and the results obtained are exceedingly satisfactory. 
The methods adopted for the accurate comparison of the coils with other wire 
standards and with mercury standards of resistance are published elsewhere,* and it 
is not necessary to describe them here. All the resistances were frequently compared 
with standard manganin coils which are hermetically sealed and the secular changes 
in which are exceedingly small. 
The new mercury standards of resistance of the National Physical Laboratory are 
nine in number. They have spherical end vessels 4 cm. in diameter and were made in 
accordance with the specificationt of the London Conference on Electrical Units and 
Standards 1908. The resolution relating to the international ohm is as follows :— 
“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 gr. in mass, of a 
constant cross-sectional area and of a length of 106'300 cm.” 
Mercury standards of resistance have also been made at the Physikalisch-Technische 
Reichsanstalt and the Bureau of Standards at Washington. Recent comparisons 
show that the units of resistance so derived agree with the unit derived at the 
National Physical Laboratory within about 2 parts in 100,000. The exact figures 
are not yet to hand. 
Section 22.—Setting of the Coils to be Coaxial with the Shaft and at 
APPROXIMATELY EQUAL DISTANCES FROM THE BRUSH CONTACT CIRCLES. 
(l) Setting of the Axes of the Coils to he Parallel to the Axis of the Shaft. 
Mechanical Method. —At the time of turning the marble cylinders, a phosphor- 
bronze ring was let into one end of each, and the surface turned at right angles to 
the axis. When the cylinders were in position on the cradles of the Lorenz 
apparatus, a radial arm supporting a micrometer head was clamped in a suitable 
position on the shaft and contact between the micrometer screw and each of the rings 
in turn was made at three points 120 degrees apart. The making of a contact was 
* ‘ B.A. Elect. Stands. Committee Report,’ 1906. 
t ‘B.A. Elect. Stands. Committee Report,’ 1909. 
