RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
89 
The effect of displacing the coils parallel to the shaft is diagrammatically shown in 
fig. 21 (b). The area OB'CS is greater than the area OLB—RNB and the difference 
of potential between O and B' is greater than that between O and B. The relation 
between the mean of these voltages and that obtained when the coils and shaft are 
coaxial will be seen presently. 
A similar change in the difference of potential between B and B' is produced by a 
rotation of a coil about a vertical axis passing through its centre. Fig. 21 (c) shows 
Fig. 21. 
generally the effect produced and makes it clear that the mean diametral plane of the 
coil may be set parallel to the brush contact circle by observations of the differences 
of potential at such points as BB'. 
The first coils to be set coaxial with the shaft by these electrical methods were 
those on cylinders Nos. 1 and 4. Afterwards the mutual inductance of the No. 1 
coils and the brush contact ch’cles was made equal to that of the No. 4 coils and the 
same circles by passing a current in the same direction through both and altering the 
distance between the No. 1 coils and the disc nearer to them until on reversal of the 
current in the coils there was no change in the difference of potential between the two 
contact circles. The object of this procedure was to enable us to measure directly 
the amount by which a potential difference such as OB'CS, fig. 21 ( b ), exceeded 
the normal potential difference such as OB'C, fig. 21 (a). The circuit formed will be 
YOL. CCXIV.-A. 
N 
