OF ALL THE PHENOMENA OF TERRESTRIAL MAGNETISM. 
103 
power in itself could be conceived capable of producing this effect, the de- 
ductions above made, although they would have stood incontrovertible, yet 
the causes of them would have remained inexplicable, but for the important 
discovery of Mr. Oersted, which threw a new light upon magnetic investi- 
gations. 
This philosopher discovered that a wire conducting an electric current was, 
during the interval of transmission, in a state of magnetic induction. Such a 
discovery, at such a time, was most fortunate ; and not on this point only, but 
on various others, it excited the attention of all the most distinguished philo- 
sophers of Europe. The number of interesting facts which were thus elicited 
will always form a prominent feature in the scientific history of the nineteenth 
century, but the greater part of them are unconnected with the present inquiry; 
I shall therefore only refer to one or two, which have an important bearing 
upon the question under investigation. As soon as I was informed of this 
interesting discovery, I was anxious, by experiments as nearly similar, as cir- 
cumstances would admit, to those I had adopted with the iron ball, to elicit in 
this case also the laws which govern the reciprocal action of the wire and the 
needle ; and after a pretty long series of experiments I arrived at this conclu- 
sion, viz. — 
“ That the force of each particle in the wire on each particle of the needle 
varies inversely as the square of the distance, and that the nature of the force 
is tangential, that is, such as would place a needle, neutralized from the earth’s 
magnetism, always at right angles to the direction of the wire, and to the 
direction of the line joining the needle with the centre of action of the wire.” 
This law, with an account of the experiments from which it was derived, was 
read to the Royal Society, May 22nd, 1822, and was afterwards published in 
the. second edition of my Essay above referred to. 
While I was thus engaged in endeavouring to elicit the law of action be- 
tween the wire and the needle, M. Ampere had entered upon a much more ex- 
tensive investigation; that is, not only of the reciprocal action between the wire 
and the needle, but also between different wires and galvanic currents on each 
other. Galvanic needles, both dipping and horizontal, were constructed, which 
possessed all the properties of the usual magnetic needles. The law of action 
of galvanic currents on each other was reduced to that of attraction ; and by 
