ON STANDARDS OF ELECTRICAL RESISTANCE. 353 



differing in some cases by 1-6 per cent. The same degree of accuracy was 

 obtained in the determination of the mercury unit by Dr. W. Siemens, de- 

 scribed in ' Phil. Mag.' 



On the other hand, in the experiments to be described, and in those made 

 by Mr. Sabine, the results differ by only a few hundredths per cent. 



The results of the determinations of the conducting-power of the gold- 

 silver alloy, described in the ' Phil. Mag.' Eeb. 1861, differ from each other 

 by 1-5 per cent., the values now found for the same quantity differ by only 

 seven-hundredths per cent. No doubt if greater care had been taken and 

 more perfect instruments used, stiU better results would have been obtained. 



Perhaps the great difference between what is above called great care and 

 ordinary care lies in the time employed. The experimenter using great care 

 has to neglect almost all consideration of time, and repeat his experiments at 

 reasonable intervals, in all cases in which it is possible, that by lapse of time 

 such error, as at first there is no means of detecting, may increase, and so 

 become apparent. The meaning of absolute care is clear. AMien absolute 

 care is taken no precautions are omitted, the best instruments obtained, and 

 every care taken in the manipulation. 



The apparatus used in the following research will first be described, the 

 results obtained will be then given, and finally some remarks made on them. 



Description of Appabatus. 



Battery. — The battery employed was a single Bunsen's cell. The wires 

 connecting it with the bridge ran parallel to each other the whole of their 

 length, so that no attraction was exercised on the magnet of the galvano- 

 meter by the current traversing them*. 



Balance. — For measuring the resistance of the wires a Whcatstone's ba- 

 lance, as modified by Kirchhoff, was employed. A plan of it is given at 

 Plate IV. (fig. 1). 



L and K are two resistance coils acting as the arms of the balance. They 

 are joined by the wire A A^, along which the eblock B connected with one end 

 of the galvanometer coil can be moved. 



The wire A A' of the instrument was made of an alloy containing 85 per 

 cent, of platinum and 15 per cent, of iridium. The advantages of employing 

 this alloy are that it does not readily oxidize, that it does not change much 

 in conducting-power with an alteration of temperature, and that it does not 

 alloy with mercury. 



S is a standard coil immersed in an oU-bath. 



P is the wire to be measured or compared with the standard S, and is 

 immersed in a large trough of water. 



G is an ordinary galvanometer by which approximate results are first 

 observed. 



Gj is a very sensitive Thomson's reflecting galvanometer, by which the 

 final observations are made. 



Mj, Mj &c., wi,, »)(, &(?• are mercury cups used to connect the several parts 

 of the circuit by thick copper rods and bars, ])lainly shown on the drawing. 

 The arrangement shown was foimd convenient, as it admitted of adjustment 

 to various positions and dimensions of conductors to be compared. The 

 position of B on the wire A A' could be observed by a boxwood scale divided 

 into millimetres and a pointer on the block. 



K is a key used to complete tlie batteiy circuit, and worked by a treadle 



* Tlie batteiy circuit was generally broken, and was closed by pressing down a treadle, 

 placed under the table, with the foot. The terminals were of platinum. 



