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VII. Experiments, by the Method of Lorentz, for the further Determination of 
the Absolute Value of the British Association Unit of Resistance, with an Appendix 
on the Determination of the pitch of a Standard Tuning-Fork. 
By Lord Rayleigh, F.R.S., Cavendish Professor of Experimental Physics in the 
University of Cambridge, and Mrs. H. Sidgwick. 
Received December 8, 1882,—Read January 11, 1883. 
§ 1. In this method, which was employed by Lorentz in. 18 73,'" a circular disc of 
metal is maintained in rotation at a uniform and known rate about an axis passing 
through its centre, and is placed in the magnetic field due to a battery current which 
circulates through a coaxal coil of many turns. The revolving disc is touched at 
its centre and circumference by two wires. If the circuit were simply closed through 
a galvanometer, the instrument would indicate the current due to the electromotive 
force of induction acting against the resistance of the circuit. The electromotive force 
corresponding to each revolution is the same as would be generated in a single turn of 
wire coincident with the circumference of the disc by the formation or cessation of the 
battery current. If this be called y, and M be the coefficient of induction between 
the coil and the circumference, m the number of revolutions per second, the electro¬ 
motive force is mMy. In the actual arrangement, however, the circuit is not simply 
closed, but its terminals are connected with the extremities of a resistance R, traversed 
by the battery current, and the variable quantities are so adjusted that the electro¬ 
motive force Ry exactly balances that of induction. When the galvanometer indicates 
no current, the following relation, independent, it will be observed, of the magnitude 
of the battery current, must be satisfied— 
R=mM ; 
and from this, M being known from the data of construction, the absolute resistance 
R of the conductor is determined. 
One of the principal difficulties to be overcome arises from the smallness of the 
resistance R, necessary for a balance, even when m and M are both increased as far 
as possible. Lorentz employed three resistances, ranging from ’0008 to '002 of a 
mercury unit, and he evaded the necessity of comparing these small resistances with 
ordinary standards by constructing them of actual columns of mercury. His result 
was accordingly obtained directly in terms of mercury, and was to the effect that 
* Pogg, Ann., cxlix., p. 251. 
