378 PROFESSOR KNOTT ON SOME RELATIONS BETWEEN 



The experiments were conducted in exactly the same manner as described in my 

 earlier paper. The wire hung vertically within a magnetising solenoid, and to its lower 

 and free end was attached a dipper, which dipped into a pool of mercury and made the 

 necessary contact for the current along the wire. The tension in the wire was varied 

 by putting on and taking off ordinary lead weights. The twist was measured by means 

 of a mirror which was fixed to the free end of the wire, and reflected a spot of light in 

 the customary manner upon a scale. 



When the wire was being traversed by a steady current so as to be circularly 

 magnetised, the longitudinal magnetic field in which the wire was placed was repeatedly 

 reversed until a steady difference of readings on the scale was obtained. The wire was, 

 in other words, subjected to a cyclic straining, which gave a total to-and-fro twisting of 

 the wire. For any one condition, some six or eight distinct successive readings were 

 noted down, so that a good mean for the total cyclic twisting of the wire was obtained. 

 Half the amount of this total twist was taken as the twist corresponding to the particular- 

 combination of magnetising forces. 



With a given current along the wire, the current in the magnetising helix was taken 

 through an ascending series of values. The highest field so. obtained usually lay between 

 25 and 35 (C.G.S.) electromagnetic units. In the case of the iron wires, this greatest field 

 was considerably higher than the field for which the maximum twist was obtained. In 

 the case of the nickel wires, there was no such maximum twist observed up to the fields 

 employed.* This point of maximum twist is one which offers distinct facilities for the 

 discussion of the problem in hand, namely, the effect of tension on the phenomenon of 

 twist. Does change of tension cause a change in the position of this maximum twist, and 

 does it cause a change in its amount ? 



Some 120 distinct curves were obtained for different iron wires, showing the relation 

 between twist and field for different currents along each wire and for different tensions. 

 If we compare the curves for any one steady current along any one wire, we shall be able 

 to study the direct influence of tension on the Wiedemann effect. The first conclusion is, 

 that there is no evident relation between the tension and the position of the maximum; or, 

 more .accurately, that the field, which for given current along the wire corresponds to the 

 maximum twist, is in no way affected by change of tension through a considerable range. 

 On the other hand, as was also clearly shown in my earlier paper, it is abundantly evident 

 that the position of the maximum twist depends on the strength of the current along the 

 wire — the stronger this current, the higher the field needed to produce the maximum 

 twist. 



The effect, however, of tension upon the amount of twist is very marked. Thus, if 

 we take any iron wire, and subject it at different tensions to the same combination of 

 circular and longitudinal magnetic stresses, we shall find that the twist due to this com- 



* In more recent experiments, conducted on a somewhat different plan, I have heen able to obtain a maximum 

 twist in nickel for intermediate values of field ; but much higher fields must be used than were available in Mr 

 Nagaoka's experiments. 



