2382 Mr. R. Appleyard on the Conductometer. 
moved, 7. e. the deviation to be applied, to compensate for 
1 mil, or for any fraction of 1 mil difference of diameters of 
the two wire s, for any value of d within the range. Provided 
that y and dare in the same unit, the expression for L is 
pertectly general. 
These partial operations (A) and (B), when combined, re- 
present the complete corrections to be applied within the 
required limits of accuracy. I have proved that the errors 
involved by carrying out this method for a range from 95 to 
105 per cent. conductivity never exceed 0:1 percent. In the 
conductometer, the two partial operations are carried out by 
setting two scales with reference to the electrical middle of 
the bridge- wire. The first scale, marked “ 10 ” at the 
middle, is divided into say 10 divisions, each ,1, of the 
length ‘of the bridge-wire, every such division representing 
1 per cent. conductivity. This scale is set to correspond 
with the conductivity of the standard wire, as above explained. 
Or the standard wire may be replaced by a resistance-coil of 
the same material, corresponding to the resistance of a wire 
of Jength equal to that of the test-wire of diameter d, and of 
say 100 per cent. conductivity, in which case the “100” 
mark of this scale is placed in coincidence with the middle of 
the bridge-wire, and is there clamped so long as that kind 
of wire is being tested. A second scale is divided into gra- 
duations each equal to +4; of the total length of bridge- 
wire. The middle point of this scale is marked “0.” A 
sliding contact for the bridge-wire can be set and clamped to 
this scale, at a _ point along ‘it corr esponding to the deviation, 
as above explained. This setting is to right or lett of the ‘ 0,” 
according to whether the test-wire is of greater or of less 
diameter than the standard wire. The deviation can be 
calculated from the mass, as well as from the diameter of the 
wire ; the appropriate scale-setting is then given simply by 
‘ fifty times the difference of the masses, divided by the sum of 
the masses,” for any equal lengths whatever of tesi-wire and 
standard wire, the masses being expressed in any single unit 
of mass whatever. For routine testing, where large quantities 
of copper have to be dealt with, the average time of a test for 
a long series of tests has by this instrument been reduced to 
18 seconds, with the diameter method. The theory and the 
mechanical details of the instrument are fully described in 
the ‘ Proceedings of the Institution of Civil Engineers,’ 
vol. cliv. Session 1902-1903, part iv. It is clear that the 
principle above described for compensating for differences of 
diameter can be applied to potentiometer work generally. If 
conductivity is assumed constant, the instrument can be used 
as a very sensitive micrometer. 
