Properties of the Alloys 0/ Iron and Aluminium. 123 



the permeability for an alloy containing 10 per cent, of 

 aluminium is reached at about —90° C. 



(5) An alloy containing 18*47 per cent, of aluminium has 

 a critical point of about 25° C, and gives no indication of 

 temperature hysteresis. This alloy probably has a maximum 

 permeability at a temperature much below —90° C. 



§ II. The Magnetic Measurements. 

 Theory of the Method. 



The ring of metal X (fig. 1) whose magnetic properties 

 are to be investigated is wound with a primary P 2 and a 

 secondary S 2 . The primary is connected in series with a 

 battery B, the primary P : of a standard mutual inductance 

 coil Y, an adjustable resistance R, a reversing-key K, and a 

 Weston's ammeter A. 



The secondary S 2 is connected in series with the secondary 

 Si of the standard mutual inductance-coil Y, and two adjust- 

 able noninductive resistances R x and R 2 . 



One terminal of a galvanometer is connected with the point 

 a between R x and R 2 , and the other terminal with the point 

 (S on the opposite side of the bridge. 



The secondaries are connected up so that the flows gene- 

 rated in them on reversing the primary are opposed in the 

 branch a /3. 



The resistances H x and R 2 are adjusted until on reversing 

 the primary circuit no quantity of electricity passes through 

 the galvanometer. 



Then, if . ' 



B = the induction in the ring X, 

 <r=the mean sectional area of the ring, 

 N = the number of turns in the secondary of ring (8 2 ), 

 r 2 = the resistance of S 3 , 



M = the mutual inductance ot Y, 

 7= the value of the steady current in the primary 

 circuit: 



