43 
RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
The apparatus was astatic and our experiments were directed to eliminating trouble 
due to thermo-electric effects at the brush contacts. 
The first brushes experimented with were of fine phosphor-bronze wire, each brush 
consisting of about 50 wires bound together after the manner of a common pencil 
brush. A light spiral spring ensured contact between a brush and the edge of a disc. 
A circuit through brushes, discs, and a galvanometer was completed, and the discs 
(25 cm. in diameter) were run at a speed of about 1500 revolutions per minute. 
Whatever thermo-electric effects existed at the points of contact of the brushes 
on one disc must have been in opposition to those at the brush contacts on the 
other disc, but the galvanometer deflection was far from steady, the variations in the 
thermo-electric effects amounting to about O'OOOl volt. Although these preliminary 
experiments of ours were so unsatisfactory, the results were much better than those 
obtained when the circuit was completed through brush contacts at the edge of one 
disc and others on the shaft of the apparatus. 
An even more disturbing feature of our experiments was the inconstancy of the 
resistance of the galvanometer circuit. With one brush on each disc the resistance of 
the contacts was measured as something less than 0’1 ohm when the discs were 
stationary, but when the discs were rotating the contact resistance increased to 5, 10 
and 20 ohms, and at times the circuit appeared to be broken. This effect was 
undoubtedly due to a vibration of the brushes brought about by the motion of the air 
at the edges of the discs. In practice it was not possible to prevent this by increasing 
the pressure applied to a brush, as by doing so the discs were worn away very rapidly 
and the points of contact became very hot. Shielding devices were tried with some 
small success, but the most favourable results were obtained by employing a number 
of brushes in parallel and placing in series with each brush a resistance of 10 ohms, 
thus ensuring that any increase or diminution of the resistance of one brush contact 
would have but little effect on the circuit as a whole. Our experimental results 
showed clearly that the employment of a large number of brushes was beneficial, but 
an increase in the number did not greatly improve the results, unless a resistance was 
placed in series with each before the brushes associated with any one disc were placed 
in parallel. 
A further advance was made by lubricating the edges of the discs. Fatty oils and 
greases are impossible for such a purpose as the contact resistances are enormously 
increased and the general result is far worse than when no lubricant at all is used. 
Acheson graphite, aquadag and oildag were tried, but the results were not satis¬ 
factory. It is not very well known that paraffin oil improves all ordinary contacts 
such as those associated with slide wires, plugs, &c., but throughout the National 
Physical Laboratory paraffin oil is largely used for such purposes. We tried it there¬ 
fore as a lubricant for the brushes. There was a marked improvement in the results, 
and we continued to use it in this preliminary work. Many kinds of brushes were 
tried : some were wires of phosphor bronze, some of phosphor-bronze gauze, some of 
G 2 
