42 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
in the disc, the wires pass along a groove G, milled in the stabilit, and are soldered to 
ten phosphor-bronze wire segments, of square section, which are screwed to the stabilit 
in the manner shown in the illustration. This method was adopted because of the 
strain to which the segments are subjected when the machine is running at full speed 
and which results in an increase in the effective diameter of a disc, measurements of 
which are given in Section 16. The wire for the phosphor-bronze segments is 25 sq. mm. 
in cross section and was kindly drawn for us by the London Electric Wire Company. 
Some of it was further drawn down into circular section wire of 0'12 mm. diameter, 
and this latter was employed for the brushes. Phosphor bronze was chosen for the 
segments and brushes because of the negligible tendency to bind and tear when this 
alloy is employed for moving parts in contact. 
Fig. 5 shows five segments in position on a disc and illustrates the manner in which 
the wires pass from the segments to the main shaft. Before making any resistance 
measurements the shaft was rotated slowly, and with the aid of a small portable 
milling machine a smooth surface was turned on the segments. The sharp edges 
which resulted at e, e (fig. 5) were rounded in order to avoid a cutting action on the 
wire brushes. 
Section 7.—The Brushes and Their Lubrication. 
To obtain experience regarding the best form and number of brushes, preliminary 
apparatus consisting of two rotating discs of phosphor bronze was constructed in 1908. 
