RESISTANCE BY A METHOD BASED ON THAT OF .LORENZ. 
35 
Laboratory workshops, and Sir Andrew Noble, F.R.S., was approached with a view 
to this part of the instrument being made in the workshops of Sir W. G. Armstrong, 
Whitworth & Co. at Elswick. Sir Andrew Noble not only undertook that this 
should be done, but took such a keen interest in the work that he generously provided 
it at very much less than the cost price. 
In preparing the plans we had full access to the working drawings of the McGill- 
Lorenz apparatus made for Prof. Callendar by Messrs. Nalder Bros. & Co. 
Although we were unable to take much advantage of this opportunity owing to the 
big difference in the designs of the two instruments, we desire to express our thanks 
to Messrs. Nalder Bros, for their kindness. 
The instrument herein described possesses many new features of importance. It is 
permanent, but determinations of resistance made from time to time will not assume 
constancy of any dimensions ; it is comparatively easy to use, and the results obtained 
are believed to be of high precision. 
Section 2.—The Method op Lorenz. 
In this method, which was first employed by Lorenz* in 1873, a rotating disc is 
placed in a magnetic field produced by a current which circulates through a coil 
coaxial with the disc. In the apparatus used by Lorenz wire brushes made contact 
with the disc at its circumference and centre, and the circuit was closed by connecting 
the brushes, through a galvanometer, with the extremities of a resistance R. The 
voltage induced by the uniform rotation of the disc at n revolutions per second is 
M in, where M is the mutual inductance of the coil and a circle coincident with the 
edge of the disc, and i is the current through the coil. This induced voltage is 
balanced against the difference of potential R® between the extremities of the 
resistance R through which the same current flows as through the coil. When 
equality of voltage is indicated by the galvanometer R = wM. M is calculated from 
the dimensions of the coil and disc and thus R is found. The coil used was of a large 
number of turns in order to make M as great as possible. 
In the ‘Philosophical Magazine’ for November, 1882, Lord Rayleigh compares 
the method of Lorenz with other methods. After stating that he is disposed to 
consider Lorenz’s method the best, he proceeds to deal with some of the difficulties. 
The first of these is the smailness of the resistance R which can be directly measured, 
and this led Lord Rayleigh to adopt a system of shunted resistances which, for part 
of our work, we also have used. Lord Rayleigh remarks that the influence of 
terrestrial magnetism and the thermo-electric effects at the sliding contacts are both 
very appreciable and give rise to trouble during the observations, but they can be 
eliminated by observing only the effect of reversing the battery current. 
The more important portion of Lord Rayleigh’s comments deals with the ratio of 
* Lorenz, ‘ Pogg. Ann.,’ 149, p. 251, 1873. 
F 2 
