412 Mr. T. Gray on the Electrolysis 



balance comes nearly to zero, and then adjust by the rheostat. 

 Leaving everything in position, break the circuit and take 

 out the cathode plates, wash, dry, and weigh them, in the 

 manner already described. Again put the plates in the cell, 

 and observing the time carefully on an accurate time-keeper, 

 close the circuit. The current will at once assume almost the 

 exact strength, and what little deviation there may be can be 

 adjusted in a second or two by the rheostat. From five to 

 ten seconds usually suffices to bring the current accurately 

 to the proper strength, and the electrolysis is as a rule con- 

 tinued for an hour. Now suppose the average error during ten 

 seconds amounts to as much as five per cent., the error on the 

 total amount will only be about one-seventieth per cent., which 

 may be neglected. The current is kept steady by means of 

 the rheostat during the whole hour, and with most instruments 

 this can be done with sufficient accuracy by observing the 

 index of the instrument itself, but if the instrument is not 

 sufficiently sensitive, or if the constancy of its indication is 

 to be tested, a second instrument possessing the requisite 

 sensibility and constancy is introduced into the circuit in the 

 manner indicated at G, and the current kept steady by 

 observing the index of it. The constant of this second 

 instrument does not require to be known ; it should be of such 

 a kind that its constant can be readily changed so as bring 

 its deflection to the proper amount, and the index exactly to 

 a scale-division. The position of the index of the instrument 

 being standardized may then be observed from time to time 

 and the variations, if any, noted. 



When instruments which are designed for the measurement 

 either of very weak or very strong currents, as, for example, 

 milliampere- or hectoampere-meters, are to be standardized, it 

 is generally more convenient to send a stronger current in 

 the former case, and a weaker current in the latter case, 

 through the electrolytic cell. Several methods involving the 

 use of auxiliary galvanometers have been used for this purpose ; 

 but the method illustrated in fig. 9 is sufficient to indicate the 

 general principle of divided circuits on which such methods 

 are based. 



Referring to the figure, r, r x is a set of resistances, supposed 

 in this case to consist of eleven straight wires each of exactly 

 the same resistance and as nearly as possible of the same 

 length and thickness. These wires are soldered to thick bars 

 of copper, b, b l9 b 2 , the resistances of which are negligible in 

 comparison with that of the wires. In the case here considered 

 the resistances are supposed to be arranged in two groups of 

 ten and one respectively, but for goneral purposes it is more 



