EFFECT OF MA<JN'ETLZ VTIOX OX THE PEUMAXEXT TWlST OF XlOlvEL. 327 



;ind to the elastic after-effect would l)e mixed together. 



As a general rule, the after-effect for the same angle of twist is 

 smaller as the wire becomes thicker. For this reason, Professor 

 Wiedemann used tolerably thick iron wires. Althouo-li much cer- 

 tainty is gained as to the effect due to magnetization only Ijy usinn- 

 thick wires, yet there is the great disadvantage that the amount of 

 untwisting is ^ ery small. A\'itli nickel wires, the elastic after-effect is 

 very small, and we can use tliin wires without iiicurring tlie risk of 

 mixing the effect due to magnetization and tliat due to elastic after- 

 effect. 



A nickel wire 0.51 mm. thick and 27 cms. l<jng was kept twisted 

 through 60° for an hour, the longitudinal ])ull acting on the wire 

 being the weight of the cross before mentioiied. When released from 

 torsion, the wire had a permanent twist of 2° 38'. On the cessation of 

 the torsional oscillations, the f(j] lowing deiiections with simultaneous 

 readings of the chronometer were taken. 



Time Torsion 



3'' 19'".0 p. m. (Uth \]n-\] 1889) 2° 38'.() 



ol'.G ('remperature) 



o7'.4 0".5 



37'.2 



37'.0 

 a. m. (12th) 3(r.2 (1)^2) 



The readings show that the after-effect in nickel is very small. 

 The wire above tested would have been untwisted through a few minutes 

 mtjre, if we had waited for some weeks or months. J^oading the wire, 

 however, increases the after-effect, 1)ut when C(jmpared with the after- 

 effect in iron under similar circumstances, it is very small. It is un- 

 necessary to give the result of numerous similar experiments. Suffice to 

 say, they all lead t(j the same conclusion. The precautions which must 



