468 
PROFESSORS A. W. REINOLD AND A. W. RUCKER ON 
Example.—September 27. Film I., at 12 h 58 m tbe deflections are, for film 83 [or 
84 when allowance is made for the independent difference of potential of the needles”, 
and for resistance 159‘6. 
84 (1 - 84 X 10 -4 ) = 83-4 nearly 
159-6 (1 —159-6 X10" 4 )=157 
83'4 
Hence the resistance is — X 508,000 = 269,400 ohms. 
VIII. Method of testing the accuracy of the experiments. 
Observations with the Galvanometer. —With the object of ascertaining whether the 
apparatus was in satisfactory working order, the following experiment was occasionally 
made. A strip of Dr. Muikhead’s carbon paper was attached to a piece of glass by 
brass clips and carefully insulated. Its extremities were connected to a Wheatstone’s 
bridge and also to the electrometer, and its resistance measured by the two methods 
in succession. Usually an observation by galvanometer was made between two 
observations by electrometer, care being taken to prevent changes of temperature of 
the carbon paper. It is unnecessary to give details of these experiments, but the 
following may be quoted as indicating the character of the results obtained :— 
Galvanometer. 
Electrometer. 
Difference. 
Per cent. 
September 24 . . 
1,620,000 
1,633,000 
0-8 
„ 25 . . 
1,455,000 
1,457,000 
0-14 
„ 26 . . 
1,910,000 
1,920,000 
0-52 
The differences in the resistance of the carbon paper from day to day were due 
partly to differences of temperature, but chiefly to changes in the hygrometric state of 
the atmosphere. 
The experiments with the carbon paper were repeated at intervals. If the difference 
of the resistances obtained exceeded 1 per cent, the observations on the films were 
interrupted, until, the apparatus having been overhauled, the two methods gave results 
with the carbon paper in sufficiently close agreement. A similar experiment was 
sometimes performed with the films themselves. The specific resistance was calculated 
both from measurements made on a portion of the film by the electrometer, and also 
from the resistance of the whole length measured by Wheatstone’s bridge. To effect 
this, the thicknesses at top and bottom, and, if necessary, at intermediate points, were 
measured. 
These observations confirmed our opinion of the superiority of the electrometer 
