﻿Electrical Resistance of Xylol- Alcohol Mixtures. 6C9 



The resistances may be made with commercial xylol and 

 absolute alcohol which has been distilled from lime. The 

 xylol possesses no appreciable conductivity, and no useful 

 purpose is served by drying the alcohol further. The con- 

 ductivity of the mixture, when the proportion of alcohol 

 does not exceed some 12 per cent., increases much more 

 rapidly than the concentration of the alcohol. When the 

 proportion of alcohol exceeds some 12 per cent., the mixture 

 ceases to obey Ohm's law accurately. The most convenient 

 mixture to use contains 1 part of alcohol to 10 parts of xylol. 

 The specific resistance of this mixture is of the order 

 10 10 ohm. A resistance suitable for electrostatic mea- 

 surements should generally not exceed 10 12 ohms; a resist- 

 ance of that magnitude can be made conveniently from a 

 piece of quill tubing about 15 cm. long bent into a U-form. 

 Electrodes can be made of copper or zinc as well as of 

 platinum. Of course the liquid must be hermetically sealed 

 in glass, if the resistance is to be constant. 



Unfortunately the resistance of the mixture has a large 

 temperature coefficient. Between 15° and 30° the resistance 

 ac a temperature t is given by 



R^Raojl 4 0-014(^-20)}. 



A temperature lower than 15° should not be used ; it was 

 found impossible to obtain consistent readings below that 

 temperature. 



The liquid resistances appear to me to have every advantage 

 over " Bronson " resistances, except in the matter of sensi- 

 tiveness to change of temperature. In most experiments 

 ample accuracy can be attained by correcting for the tem- 

 perature, while if extreme accuracy is required it is easy 

 to maintain a constant temperature in a thermostat. The 

 resistance can be determined readily in absolute measure by 

 comparison with a wire megohm. For this purpose a series 

 of resistances ranging from 3 x 10 7 ohm to 10 12 ohm were 

 made, each resistance being about 15 times greater than the 

 next lower. The comparison is easily effected by a Wheatstone 

 bridge in which a sensitive electrometer (3000 divisions for a 

 volt) is substituted for the ordinary galvanometer. The 

 points of the bridge which are usually connected to the 

 galvanometer are connected one to earth and the other to one 

 pair of quadrants of the electrometer, of which the other 

 pair of quadrants is earthed ; the other arms of the bridge 

 are taken from a Post Office Box. The resistance between 

 the battery and earth must be large compared to that in the 

 Post Office Box. 



