382 Trouton — The Results of an Electrical Experiment, involving the Relative 



begin with, and to see if its rate of decrement was increased by the effect 

 in question when properly timed to do so. No effect was ever observed. 



The Sensitiveness of the Apimratm. 



The method adopted to test the sensitiveness of the apparatus to small 

 quantities of energy, supplied synchronously with its own period, was to place 

 a small magnet on the suspended apparatus at its centre, and to arrange that an 

 intermittent magnetic field was synchronously a^Dplied for half the period of 

 swing. The field was applied by means of a circuit of wire placed parallel to the 

 magnet and at one side. 



To ascertain the minimum magnetic force competent to set up a swing, the 

 procedure was to gradually reduce the current in this circuit of wire until 

 no effect could be observed on the apparatus at rest, the current all the while 

 being applied at the j^roper intervals. From this a maximum limit to the value 

 to be assigned to the minimum energy capable of producing a swing may be 

 determined, by calculating the energy given per swing by this minimum field, 

 when the apparatus had reached a swing clearly observable, because when it 

 was swinging through all angles less than this the energy supplied must have 

 been proportionally less. When the suspended apparatus swings through an 

 angle Q, the energy supplied is MHO, where 31 is the moment of the magnet, 

 and H the strength of the applied magnetic field. The minimum magnetic 

 field was found to be about H — '037 when employed with a magnet whose 

 moment was il!f=204; and taking that a movement of the spot of light of 

 2 mm. could be detected, we get for the angle 6 the value \ x 2/1860, the 

 scale being 1860 mm. from the mirror. This gives for the energy supplied once 

 in each complete oscillation 3IFd = '004 ergs about. That is to say, we may 

 safely assert that "004 ergs is competent to set ujd a swing. 



■ We have seen, however, that the energy necessary to generate the magnetic 

 field assumed to exist is as great as one erg, and this was applied and removed in 

 the experiments each half oscillation ; so that it is evident that some other source 

 for the energy or some countervailing effect must clearly be looked for. 



On the last opportunity I had of discussing the matter with Professor Fitz- 

 Gerald, preliminary experiments had been made giving as far as they went 

 negative results : the final results not being completed till after Science had 

 to deplore the grievous loss it sustained at his death. Fitz Gerald, on that 

 occasion, made a remark which, as well as I remember, was to the effect that 

 should the negative results then obtained be sustained by further work, he would 

 attribute the non-occurrence of any observable effect to the same general cause as 

 produced the negative results in Michelson and Morley's experiments on the 



