38 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
their dimensions, together with those of the discs, may be determined with ease and 
with precision ; in addition, any possible error in the measurement of the diameters of 
the discs is made negligibly small. Instead of employing one rotating conductor, a 
number are used, and the induced voltages may be placed in parallel or in series. The 
machine is of the best construction, great attention being paid to lubrication in order 
to avoid irregularities in the speed. As already stated, the heavy metal work was 
kindly undertaken by Sir W. G. Armstrong, Whitworth & Co., but the smaller work 
such as the chronograph, commutators, brushes, coil fittings, &c., many of which 
demanded great skill in their construction, were made by Mr. F. H. Murfitt, who 
has charge of the instrument shop of the Laboratory, and whom we also thank for 
many suggestions. 
Section 3.—General Description. 
The instrument consists of two metallic discs which support ten conducting wires 
rotating in a magnetic field produced by a current in four coils ; an electric motor is 
used as the source of power. Phosphor-bronze wire brushes make contact with 
segments made of the same alloy attached to the ends of the rotating wires, and the 
difference of potential between the brush contacts at the ends of a single wire, or that 
between five wires, is balanced against the difference of potential between two points 
on a standard resistance, the current through which is the same as that flowing through 
the four coils. A diagrammatic sketch of the arrangement is shown in fig. 1. 
It will be seen that the current in the coils 1 and 2 is in the opposite direction to 
that in the coils 3 and 4. The resulting magnetic fields are opposed in direction, and 
the value of the field at points in the neighbourhood of the edge of a disc is zero 
or nearly so. A difference of potential is produced between the ends of a rotating 
conductor, and its value is dependent on the position only of the ends of the 
