G4 Mr. F. J. Smith on the Measurement of the Fall 



cannot be taken as proving that the theory is wrong, 

 and that the forces ought not to be observed, bnt it does 

 clearly show that they must be excessively minute, and that, 

 if they exist, it is only by great refinement that they can be 

 made evident. No doubt, as Prof. Fitzgerald suggested, the 

 use of concentrating cylindrical mirrors of proper form would 

 greatly increase the sensitiveness of the whole apparatus. 



We have been prevented from completing this work be- 

 cause the elementary laboratory was required, and it is im- 

 possible to carry on any really delicate research requiring 

 uniform conditions and steadiness in so unsuitable a building 

 as the Science Schools. We have therefore published our 

 results in their present imperfect condition, partly because we 

 believe that the methods employed may be useful to others, 

 but chiefly because we hope that some physicist who has the 

 advantage of having a steady underground room may be 

 induced to make the experiments under more perfect con- 

 ditions, and so carry the matter to a successful issue. 



VI. The Measurement of the Time of the Fall of Magnetization 

 in a Magnetized Iron Cylinder. By Fkedeluck J. Smith, 

 M.A.y Millard Lecturer on Mechanics and Fhysics, IVinity 

 College, Oxford' 



* 



WHILE working at the construction of electromagnetic 

 styli, as used in the Tram Chronograph, described at 

 p. 377, Phil. Mag. xxix., I wished to determine the con- 

 ditions for producing a stylus with a small " latency." One 

 of the principal things to be determined was the time of the 

 fall of magnetization in different iron bars. As the deter- 

 mination of this period of magnetic change appears to be a 

 subject of interest apart from the way in which it affects the 

 action of styli, I beg respectfully to send you a brief account 

 of the method by which I have arrived at certain results 

 respecting it. 



If an iron bar be magnetized by a current of electricity 

 and the current be broken, the bar after a short time loses 

 some of its magnetism. The time varies with the kind of iron 

 used and the magnetizing force. 



If the terminals of the coil conducting the magnetizing- 

 current could be quickly shifted from the battery to the ter- 

 minals of a galvanometer, then a deflexion would be expected 

 if the pole changing took place before the magnetization of 



* Communicated bv the Author. 



