RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
45 
The brushes are of phosphor-bronze wire O'12 mm. in diameter, and to obtain as 
small a thermo-electric effect at the contacts as possible the wire was drawn from 
other wire of square section similar to that employed for the segments on the discs. 
To make a brush, the fine wire is wound in screw cut grooves of 0'25 mm. pitch, cut 
on small brass cylinders which can rotate about, or be clamped to, the axles AA'. 
The ends of the wire are soldered to the cylinders and the wire brush thus formed is 
put in tension by operating the milled heads HH. The spiral springs SS' are of stout 
phosphor-bronze wire and are soldered to brass rods RR', of square section, which 
slide in square cut grooves cut in the frame F. The overall length of a brush is 
20 cm. In practice we found the tension required for good working to be very small; 
a tension corresponding to the pull produced by the suspension of a 200 gr. weight 
was found to be most satisfactory. Some of the brushes were made with eight 
turns of wire, some with three turns, and a few with one turn; the majority were of 
three turns. 
From the moment we commenced to use this style of brush with petrol as a 
lubricant, the thermo-electric variations produced very much less trouble. The 
variations are not more than one-fiftieth of those found with any other form of brush 
we have experimented with, and the variation in the resistance of the contacts when 
ten brushes are placed in series is so small that the sensitiveness of the galvanometer 
remains constant within the limits of error of our measurements. However, in the 
case of ten brushes in series, the tension on the wires has been somewhat greater than 
that recorded above. 
Without a lubricant the brushes are not satisfactory. The wire is gripped by the 
disc and under certain conditions the brushes vibrate longitudinally and produce at 
the same time a chattering noise. Also the wire and disc become rugged and the 
temperature at the point of contact is very much greater than when petrol is used as 
a lubricant. We believe the petrol to have three beneficial influences : (l) as a 
lubricant; (2) as a cooling agent; (3) as a cleanser getting rid of all traces of grease and 
dirt. The amount of petrol to be supplied and the manner of supply was the subject of 
many experiments, but it is sufficient to state here the manner eventually adopted. 
Fig. 5 shows a brush in position and a petrol supply vessel V. The brush is 
secured to a massive phosphor-bronze ring R (see figs. 3 and 5) by the screws SS', 
and it can be placed in contact with a segment or removed from such contact by 
loosening the terminal head H and giving the brush frame a circular motion about the 
pivot screw S'. The screws SS' and the brass distance pieces BIT are insulated from 
the ring R by means of ebonite sleeves, the latter allowing of some adjustment of the 
screws SSh The ring R is supported by the bed B (fig. 4) but is insulated from it. 
Stretched phosphor-bronze wires support the ring laterally and give it the necessary 
rigidity. 
The petrol supply vessel is of glass, the tube being drawn down to a capillary about 
0'5 mm. in diameter at its lower end. On the narrow glass tube thus formed, a 
