Elasticity of Ferromagnetic Substances by Magnetization. 41 



batteries was measured by a Siemens-Halske ammeter, which 

 was occasionally compared with a Kelvin ampere-balance. 



The present arrangement also enabled us to measure the 

 magnetic change of length of the specimen under constant 

 tension by applying a magnetic field to the specimen and 

 observing the deflexion o£ the mirror corresponding to 

 different fields. 



Our procedure in the experiment was usually as follows: — • 

 The wire to be tested was first annealed for about four hours in 

 a charcoal. fire, and then gradually cooled. To avoid surface 

 oxidation during the process of annealing, the wire was buried 

 in a fine powder of charcoal, and sealed in an iron tube, 

 which was placed in a furnace. In this way a small quantity 

 of carbon might enter into the wire, but the oxidation was 

 quite inappreciable. Variation in the diameter of the wire 

 caused by the oxidation was avoided in the above manner. 



To either end of the wire thus annealed was brazed the 

 brass rod before mentioned, and the whole was suspended in 

 position as shown in the foregoing figure. Then the mirror 

 and pulley-systems were placed in position. For avoiding a 

 continuous slight vibration of the mirror, as well as for the 

 rapid damping of the vibration due to loading or unloading 

 the wire, it was necessary that a glass rod fixed to a stand 

 should come lightly in contact with the bifilar cords near 

 the friction-wheel q. The addition or removal of a weight 

 from the pan was occasionally observed to cause a lateral 

 displacement of the image in the mirror; this was, however, 

 easily effaced by adjusting the clamp of the carriage to the 

 rod. 



The magnetic change of length under constant tension was 

 first measured. Since the hysteresis effect of tension on the 

 length change was found to be considerable, the tension was 

 varied cyclically betv 7 een zero and its maximum value before 

 beginning the experiment, in order to remove any initial 

 effects. 



According to the results of our experiments, the ascending 

 branch in the curve of elongation due to tension lies a little 

 above the descending one. In the following tables the length 

 changes under constant tension are results for the stage of 

 increasing weights, taken after several cycles of loading and 

 unloading. The usual processes of observation were followed. 

 The wire was first loaded with the smallest weight, which 

 was generally 1 kilogram ; the demagnetization by reversals 

 was then carefully effected. A series of gradually increasing 

 currents was passed through the magnetizing coil, and the 

 corresponding deflexions of the scale were noted. The wire 



