40 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
complete apparatus is diagrammatically shown in fig. 2 ; fig. 3 is a photograph of the 
instrument taken from the end farther from the motor. The overall length is seven 
metres. 
Section 4.—The Motor and Fly-wheel. 
The electric motor M, fig. 2, is coupled to a shaft supporting a fly-wheel W; the 
fly-wheel shaft is in turn coupled to another supporting one of the discs, and the latter 
shaft is coupled to a similar one supporting the other disc. The coupling between the 
fly-wheel shaft and that supporting one of the discs is an insulating one, washers and 
tubes of stabilit being used to secure good insulation. 
The electric motor is a shunt-wound one supplied by Messrs Crompton & Co., Ltd. 
The machine has four poles and has a commutator of the radial type. The shaft is of 
phosphor bronze and the motor is mounted on a base of the same alloy in order to 
reduce the quantity of magnetic material to a minimum. The stray field of the 
motor in an axial direction is surprisingly small, its intensity at a distance of 400 cm. 
amounting to 0'0006 C.G.S. units only. Our original intention was to enclose the 
motor in a double shell of soft iron, and the base of such a shell is interposed between 
the motor and its support; the results of our observations on the stray field showed 
this to be unnecessary and the idea was abandoned. The efiect of the motor on the 
mutual inductance of the coils and discs is discussed in Section 19. 
The fly-wheel is of phosphor bronze. The outer diameter is 50 cm. and the weight 
of metal in the rim is about 80 kgr. 
Section 5.—The Rotating Discs and Their Supports. 
The portion to the right of the instrument (fig. 2) consists of two similar parts, and 
only one of these will be described. Fig. 4 shows one of the parts. 
The disc D is of rolled phosphor bronze fitted on a shaft S made of an alloy of 
copper and aluminium (10 per cent, aluminium and 90 per cent, copper). The 
original intention was to have the shaft of phosphor bronze, and two such shafts were 
made but were rejected on account of their appreciable magnetic susceptibility. 
Phosphor-bronze billets having the requisite magnetic properties could readily be 
obtained, but in such cases they failed to give satisfactory results with the mechanical 
tests and were therefore unsuited for shafting. The diameter of a disc is about 53 cm., 
and that of the shaft is 5 cm. 
