524 On Mance's Method of Measuring Battery Resistance. 



galvanometer ; but variations in e (due to the changed u), 

 which would produce the same effect on the galvanometer, no 

 matter how much the resistances were balanced, have no time 

 to take place before the galvanometer circuit is broken ; and 

 then no further change has any effect. This works very well 

 in practice ; and the resistance of a cell can be really deter- 

 mined when producing a current through a definite resistance, 

 viz. a + b + c + d. This cannot be done accurately by any other 

 method that I know of. 



Measurement of any Liquid Resistances. 



The method may be applied to determine the resistance of 

 electrolytes in general. A long tube containing the electro- 

 lyte surrounded by a jacket of water at a known temperature 

 is interposed in the battery circuit d, the battery being one 

 whose resistance is small and can be depended on; and the re- 

 sistance of the two together, battery and tube, is measured. 



The tube is then removed, and the resistance of the battery 

 determined alone ; the difference of course gives the resistance 

 of the electrolyte in the tube. The tube can then be filled 

 with mercury and the measurement repeated. The amount of 

 polarization of the electrodes is of no more consequence than 

 the electromotive force of the battery, provided the gas given 

 off is not allowed mechanically to obstruct the current ; and 

 the effect of variations in its amount are reduced to a mini- 

 mum by the method just described for the battery. It is well 

 to make the tube end in a couple of globular receivers with 

 two necks, very much like Dewar's electrometer, and to plunge 

 large electrodes into these globes (see T, fig. 4). Their position 

 in the globes is not of very much consequence ; neither is a 

 bubble or two of gas on their surface ; the principal part of 

 the resistance is offered by the liquid in the tube. If a ther- 

 mometer is kept in each receiver and the liquid be allowed to 

 flow backwards and forwards through the tube once or twice 

 before observing, its temperature may be known without sur- 

 rounding it with a jacket. But of course this does not apply 

 accurately when it is required to raise its temperature much 

 above that of the atmosphere. Since the sensitiveness depends 

 on the electromotive force of the battery, it is well to use one 

 or two Grove's cells. Fig. 4 is a complete diagram of the 

 connexions, showing the arrangement for interchanging a and 

 b by means of a commutator, and also showing a key which 

 will break the galvanometer circuit at the instant required 

 and act instead of the extemporized arrangement of two keys 

 depicted in fig. 1. The lower spring carries a block which 

 presses up against a screw connected with the galvanometer, 

 except when it is forced down by the upper spring. This 



