382 Trouron—The Results of an Electrical Experiment, mvolving 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 Apparatus. 
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 applied 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 proper 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 nad 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 @, the energy supplied is 12H0, where J 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 M= 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 4 x 2/1860, the 
scale being 1860 mm. from the mirror. ‘This gives for the energy supplied once 
in each complete oscillation MP0 = :004 ergs about. That is to say, we may 
safely assert that ‘004 ergs is competent to set up 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, prelimimary 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. FitzGerald, 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 
