320 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
R being - the resistance of the rheomotor, r the other resistances in the first circuit, 
and X the resistance added by the rheostat to make the force of the current in the 
second circuit equal to that in the first. 
The resistance of one of the elements of the battery described in § 5, I have found 
to be equal to 2128 standard units. 
§ 13. 
The resistance of a standard rheomotor having been accurately determined by 
either of the processes above described, the resistance of any other rheomotor, in 
which the electro-motive force is the same, may be obtained by a still more expedi- 
tious method. The needle of the galvanometer being brought to a determined point 
when the standard rheomotor is interposed in the circuit ; if this be removed, and the 
rheomotor to be measured be substituted in its place, the number of coils of the 
rheostat, added to or subtracted from the circuit, to make the current in the latter 
case equal to that in the former, when added to or subtracted from the resistance of 
the standard rheomotor, will give that of the rheomotor to be measured. If R' be 
greater than R, R' = R -f- r ; but if R' be less than R, R' = R — r. By this simple 
process the resistances of voltaic elements of different forms, magnitudes, & c. may be 
readily compared. 
^ 14. Instrument for Measuring the Resistance of Liquids. 
We do not at present possess any accurate measures of the conductibilities of 
liquids, nor have there yet been formed any tables which show the even real order of 
their conducting powers. In the experiments having this object in view which have 
hitherto been made, the contrary electro-motive force, which generally arises when the 
electric current passes through a liquid capable of undergoing decomposition (§ 11,4.), 
has been left entirely out of consideration, and the results therefore have widely de- 
viated from the truth. By the simple instrument represented at Plate XVII. fig. 4, I 
have been able to eliminate completely this source of error, and to obtain perfectly con- 
stant results. A is a glass tube about two inches long and half an inch in internal dia- 
meter ; a portion of the tube is ground away for an inch and a quarter of its length, 
so as to leave a segment of 270° ; at one extremity of this aperture is fixed a metal 
plug terminated by a platinum plate, and at the other end is a moveable piston, termi- 
nated also by a plate of platinum, capable of being advanced to within a quarter of an 
inch from the fixed plate ; the range of its motion is thus limited to one inch, and an 
attached micrometric apparatus enables any portion of this distance to be accurately 
measured. To obtain the measure of the resistance of a liquid I proceed in the fol- 
lowing way: — I interpose in the circuit a small constant battery, consisting of about 
three elements, with the rheostat, the resistance-coils, the galvanometer, and the 
measuring tube just described. The end of the piston being a quarter of an inch 
distant from the fixed plate, I fill the intervening space with the liquid, the resistance 
