S7A' WILLIAM SIE.M1:.\S, F.R.S. 173 



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 equal to ^ unless C equals X. In order to work this instrument 



JL 



'ii wide ranges of temperature, it becomes necessary to make 

 (' variable, ami nearly equal to X. It is also obviously desirable 

 to have y very small as compared with X. Reliable observations 

 can, however, be obtained between the limits of v = 10 v' and 

 lo v = v', from which it follows that, with a fixed coil, C = lo 

 units, resistances may be measured (subject to correction for the 

 disproportion introduced by the value of y), between the limits of 

 1 and 100 unite. In adding a reserve coil of 1,000 units, the scope 

 of the instrument can be extended from 1 unit to 10,000 units. 

 Greater accuracy for resistances between 50 and 500 units 

 wnuld, however, be insured by providing a third resistance of 

 lot i units. 



PRECAUTIONS NECESSARY IN USING THE INSTRUMENT. 

 Certain precautions have to be taken to insure reliable results 

 in using the instrument. 



1. The dilute acid employed in both tubes should be of the 

 same strength, a condition which is easily realized in preparing a 

 standard solution of about 9 measures of distilled water for one 

 measure of chemically pure sulphuric acid ; to be kept in a bottle 

 for replenishing the instrument when required. The moveable 

 reservoirs being closed by a cork, with but a small hole for the 

 admission of air, will rarely require replenishing. 



2. When the instrument has been refilled or has not been used 

 for some days, it is advisable to verify the equality of resistance 

 of both voltameters and their connection by passing the battery 

 current through them for some minutes with equal resistances 

 inserted in each branch. If a difference between the volumes of 

 gases should be observed, the binding screws and the pads of india- 

 rubber closing the tubes should be examined and the experiment 

 r -prated. It is possible that an irregularity may be observed in 

 the first trial, owing to a difference in the condition of polarity 

 between the two seta of electrodes, which will disappear when 

 both shall have been subjected to reversed currents proceeding 

 from the same battery ; the solutions will, moreover, be fully 

 and equally saturated with gases, and absorption of the gases 

 avoided. 



a. The battery power used should be proportional to the re- 



