S70 Mr Barlow’s Electro-Magnetic Experiments. 
in its position, may be used as a constant indicator of the 
strength of; the battery. But as the application of this mea- 
sure to computation is involved in principles not at present ex- 
plained, it will be proper first to inform the reader of the means 
which I employ in the first instance to preserve the uniformity 
of action during every separate course of experiments. These 
were as follows 
“ The vessel which contains the dilute acid, into which the 
plates are immersed, holds nearly twenty gallons ; and I begin 
the experiments with little more than twelve gallons ; moreover 
the plates are not, in the first instance, let down to their lowest 
point. The intensity shown by the standard compass after the 
connection has been made some minutes, is noted ; and by 
breaking off and making the contact anew, this same intensity 
occurs again, the power being always strongest when the con- 
tact is first made ; then when the standard compass returns to' 
its former bearing, the observation with the other compass is 
taken; the contact broken, and renewed, and so on as long as 
the battery retains sufficient power. When this fails, the plates 
are lowered a little more, the power thus increased, and the ob- 
servations resumed, till at length the plates being wholly down, 
and the power too weak, recourse is had to a supply of more 
dilute acid ; by which means a tolerably steady action is kept 
up longer than is necessary for any series of experiments of this 
kind.” 
By means of this apparatus, Mr Barlow performed a series of 
experiments, in order to establish, the law of the phenomena, 
and all his results harmonise in a very singular manner with 
the general principle, M that every particle of the galvanic 
fluid in the conducting wire acts on every particle of the mag- 
netic fluid in a magnetised needle , with a force varying inverse- 
ly as the square of the distance ; but that the action of the 'par- 
ticles of the fluid in the wire is neither to attract nor to repel ' 
either poles of a magnetic particle, but a tangential force which 
has a tendency to place the poles of either fluids at right angles * 
to those of the other ; whereby a magnetic particle , supposing it 
under the influence of the wire only , would always place itself 
at right angles to the line let fall from it perpendicular to the 
wire , and to the direction of the wire , itself at that point 
