368 Mr. A. C. Jolley on the Magnetic Balance 



Superior and Rheotan behave like steels in the apparatus 

 and probably contain considerable quantities of iron in their 

 composition. 



The magnetic behaviour of Manganin is remarkable, it 

 being, with the exception of the last two alloys and Eureka, 

 the most magnetic of all the materials examined. This is of 

 particular interest in view of its almost universal adoption 

 for the construction of accurate resistances, but it is not 

 altogether unexpected when we remember that the remark- 

 able Heusler alloys have a Copper-Aluminium Manganese 

 composition. 



The figures in the rest of the table indicate the variation 

 in the value of K, with increase in mass of the specimen and 

 increase in length. In the first case the sample, in the form 

 of a bare wire 1 mm. in diameter, was cut up into standard 

 length (approx. 2*8 cms.), and successive lengths of measured 

 mass were introduced into the tube until it would hold no 

 more ; it will be observed that the value of K tends to di- 

 minish as the mass increases. (Curve A, Extra Prima, fig. 7.) 



The apparatus is, of course, very sensitive to the length of 

 specimen, and an extreme range of length was taken in a 

 wire of 1 mm. diameter, and the results are shown in the 

 curve B (copper). 



The difficulty of obtaining non-magnetic copper has already 

 been referred to, and every instrument-maker knows how 

 difficult it is to wind a really non-magnetic D'Arsonval coil. 

 In order to see how far this is dependent upon surface 

 conditions a sample of copper was taken and treated with 

 concentrated hydrochloric acid and re-tested, and it was 

 found that the value of K was reduced to nearly one-half, 

 and that further treatment had but little effect. 



From these results it will be seen that there is a consider- 

 able field of usefulness for the balance. Firstly, for examining 

 the materials to be used in the construction of standard appa- 

 ratus where the permeability becomes of first importance. 

 Secondly, in grading and examining alloys whose properties 

 are materially dependent upon minute traces of alloying 

 constituents which are so difficult to estimate by chemical 

 means, and which play such an important part in their 

 electrical behaviour. Thirdly, it could well be used to 

 maintain the standard of purity in the commercial pro- 

 duction of such a metal as Aluminium, whose chief im- 

 purity is iron, the last traces of which are so difficult to 

 remove. 



