RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
49 
speed. The drum is 50 cm. long and a record lasting 80 minutes can therefore 
be made. 
Fig. 7 is a full-size reproduction of a portion of a record, taken June 26, 1913. The 
recorded speed is 1043‘96 2 revolutions per minute. The rows of dots enable the mean 
speed to be calculated with an error certainly not greater than 1 part in 100,000, and 
the speed throughout (as illustrated by the rows of dots lying in a straight line and 
not a wavy one) is wonderfully uniform. The general fluctuations of speed cannot 
be detected on the record, but their magnitude has been estimated in another way as 
explained in Section 23. 
A mercury contact is used on the pendulum of the standard clock and a relay is 
operated continuously by the current which passes. The rate of the clock is of course 
allowed for. 
Section 12.—Magnetic Tests. 
Magnetic tests were made on all the materials employed. With the exception of 
the motor, we are satisfied that the permeability of no part of the machine and the 
concrete bed on which it rests differs from unity by more than 2 parts in 100,000. 
Every part of the machine bears a distinctive number. When each part was cast, 
a lug about 5 cm. long and 5 cm. in diameter was cast with it, and after being stamped 
with the same number as the casting it was forwarded by Sir W. G. Armstrong, 
Whitworth & Co. to be tested for magnetic quality. In the case of springs, rods, 
tubes, &c., pieces were cut from these and used for the tests. 
The method of testing was similar to that employed for the parts of the current 
balance.* Soft iron wire and ferrous sulphate were used to calibrate the apparatus 
and sufficient sensitiveness was obtained to detect a difference from unit permeability 
of about 1 part in 100,000. Thus, when powdered ferrous sulphate having a 
permeability of about 1‘00044 was contained in a glass tube having the same cross 
section as most of the test pieces, the resulting galvanometer deflection was 5'2 mm. 
The test pieces of marble were of much larger cross section than the glass tube and 
the sensitiveness was correspondingly increased. 
Trouble was experienced with a number of brass rods and tubes and with the shaft 
of the apparatus. The first shaft was made of phosphor bronze and had a magnetic 
permeability of 1'006 ; in consequence this shaft was rejected. The material obtained 
for the second shaft was of an alloy of copper and aluminium (copper 90 per cent., 
aluminium 10 per cent.) and was also rejected as its permeability was about 1‘002. 
The third shaft was made with specially pure aluminium and copper in the same 
proportions as before, and its permeability differs from that of air by an amount too 
small to be detected. The billet for the shaft was prepared by The Broughton Copper 
Company, Limited, of Manchester, and we thank them for the care taken in its 
preparation. 
* ‘Phil. Trans,’ A, vol. 207, p. 475, 1908. 
VOL. CCXIV.-A. 
H 
