On the Production of Transient Electric Currents. 131 



point now under consideration ; for in the observation shown in fig. 4, 

 the initial effect of beginning to release the wire from stress, instead of 

 being nil, was in fact just that which a small amount of vibration 

 would cause. 



§ 24. The fact that sudden torsion gives a transient current along a 

 magnetised iron or steel rod was observed by Matteucci as early as 

 1858. An account of his observations is given by Wiedemann 

 (" Galvanismus," II, § 484), from which it appears that when an iron 

 rod under the influence of longitudinal magnetising force was twisted 

 like a common screw, the current flowed from south to north. The 

 direction stated is opposite to that taken, under similar conditions, by 

 the currents in my experiments. A possible explanation of this dis- 

 crepancy may be found if we suppose that in Matteucci's experiments 

 the Villari critical point had been passed, which might have been the 

 case if he applied strong torsional stress in conjunction with strong 

 magnetisation. ITor it has been shown by Sir W. Thomson,* and 

 confirmed by my own experiments, that the Villari critical point 

 comes earlier with strong than with weak stresses. With the 

 moderate stress used by me, reversal of the effects did not occur even 

 with very high magnetising forces, but it is possible that by using 

 more powerful torsion Matteucci may have brought his rods into the 

 condition which would give decrease of magnetism by oblique pull. 

 This suggestion is borne out by his observations with hard steel, of 

 which it is said that when the magnetising current was broken, the 

 transient currents produced by torsion changed their signs after the 

 residual magnetism had been partly shaken out by the first twistings, 

 showing apparently a passage through the Yillari critical point as the 

 magnetisation was reduced. 



Matteucci has also remarked that the currents become constant 

 only after several back and forth twistings. Curiously enough he 

 says that a twisted rod gives no current when it is magnetised, but it 

 is not unlikely that this (certainly erroneous) observation may have 

 been due to an accidental omission to notice the effect of the first 

 closing of the magnetising circuit after the rod was twisted. 

 Subsequent openings and closings of the circuit do give scarcely any 

 effect. 



§ 25. By using a telephone in place of a ballistic galvanometer, 

 Professor Hughes has observed the production of transient currents 

 in a twisted iron wire by making and breaking a current in a 

 surrounding solenoid, and he has described in a recent .series of 

 papersf this as well as many other closely related results. For 

 example, he placed the iron wire in the battery circuit, and connected 

 the telephone, to the external solenoid. Sounds were then obtained 



* Loc. ext., §§ 211 and 244. 



f " Proc. Eoy. Soc," vol. 31. 



