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H. A. Rowland—Absolute Unit of Electrical Resistance. 431 
Thermometers.—Accurate thermometers graduated to half 
degrees were used for finding the temperature of the standard. 
The arbitrary standard.—This was made of about seventy 
feet of German silver wire, mounted in the same way as the 
temperature was taken as 17° C. 
To obtain the accurate resistance of this standard in ohms, I 
had two standards of 10 ohms and one of 1, 100, and 1,000 
ohms. The 1-ohm, and one of the 10-ohm standards, were made 
by Elliott of London, and the others by Messrs. Warden, Muir- 
head and Clark of the same place. But on careful comparison 
I found that Warden, Muirhead and Clark’s 10-ohm standard 
was 1:00171 times that of Messrs. Elliott Bros. On stating these 
facts to the two firms I met no response from the first firm, but 
the second kindly undertook to make me a standard which 
should be true by the standards in charge of Professor Max- 
well at Cambridge.* At present I give the result of the com- 
parison with these standards, as well as some others, and also 
with a set of resistance coils by Messrs. Elliott Bros. 
Ci tat N tators except those having mercury 
connections were used, and those in the circuit whose resist- 
ance was determined were so constructed as to offer no appre- 
ciable resistance. The commutator by which the main current 
was reversed, could be operated in a fraction of a second, so 
as to cause no delay in the reversal. 
Connecting wires,—These were of No. 22 or No. 16 wire and 
were all carefully twisted together. The insulation was tes 
and found to be excellent. ean 
Inductor for damping.—This has already been described in 
my first paper on “Magnetic Permeability,” and merely con- 
sisted of a small horse-shoe magnet with a sliding coil, which 
was introduced into the secondary circuit. By moving it back 
and forth, the induced current could be used to stop the vibra- 
tions of the needle and make it stationary at the zero point. 
This is necessary in the method where the first throw of the 
galvanometer needle constitutes the observation, but in the 
method of recoil it is not necessary to use it very often. I 
refer the method of the first throw as a general rule, but I 
ave used both methods. : 
This method of damping will be found much more efficient 
than that of the damping magnet as taught by Weber, and 
practice a single movement will often bring the needle 
exactly to rest at the zero point. 
* i and as I cannot tell when the standard will 
sive: Cpe paren cioteeanrs a Secale hoping to make a more exact 
comparison in future. 
