50 
THE CANTOR LECTURE, 1890. 
a battery of a single cup, a number of helices is required; but when a 
compound battery is used, then one long wire must be employed, mak¬ 
ing many turns around the iron, the length of wire and consequently 
the number of turns being commensurate with the projectile power of 
the battery.” 
Henry, sums up his own position as follows :—“ (1). Previous to my 
investigations the means of developing magnetism in soft iron were 
imperfectly understood, and the electro-magnet which then existed was 
inapplicable to transmissions of power to a distance. (2). I was the 
first to prove by actual experiment that in order to develop magnetic 
power at a distance, a galvanic battery of 1 intensity ' must be employed 
to project the current through the long conductor, and that a magnet 
surrounded by many turns of one long wire must be used to receive 
this current. (3). I was the first to actually magnetise a piece of iron 
at a distance, and to call attention to the fact of the applicability of my 
experiments to the telegraph. (4). I was the first to actually sound a 
bell at a distance by means of the electro-magnet. (5). The principles 
I had developed were -applied by Dr. Gale to render Morse's machine 
effective at a distance." 
Though Henry's researches were published in 1831, they were for 
some years almost unknown in Europe. Until April, 1837, when 
Henry himself visited Wheatstone at his laboratory at King's Col¬ 
lege, the latter did not know how to construct an electro-magnet that 
could be worked through a long wire circuit. Cooke, who became the 
coadjutor of Wheatstone, had originally come to him to consult him, 
in February, 1837, about his telegraph and alarum, the electro-magnets 
of which, though they worked well on short circuits, refused to work 
when placed in circuit with even a single mile of wire. Wheatstone's 
own occount of the matter is extremely explicit :—“ Relying on my 
former experience, I at once told Mr. Cooke that his plan would not 
and could not act as a telegraph, because sufficient attractive power 
could not be imparted to an electro-magnet interposed in a long circuit; 
and to convince him of the truth of this assertion, I invited him to 
King's College to see the repetition of the experiments on which my con¬ 
clusion was founded. He came, and after seeing a variety of voltaic 
magnets, which even with powerful batteries exhibited only slight 
adhesive attraction, he expressed his disappointment.” 
After Henry's visit to Wheatstone, the latter altered his tone. He 
had been usin g faute cle mieux , relay circuits to work the electro-magnets 
of his alarum in a short circuit with a local battery. “ These short 
circuits,” he writes, “ have lost nearly all their importance, and are 
scarcely worth contending about since my discovery ” (the italics are our 
own) “that electro-magnets may be so constructed as to produce 
the required effects by means of the direct current, even in very long 
circuits.” 
We pass on to the researches of the distinguished physicist of 
Manchester, whose decease we have lately had to deplore, Mr. J. P. 
Joule, who, fired by the work of Sturgeon, made most valuable contri¬ 
butions to the subject. Most of these were published either in 
Sturgeon's “ Annals of Electricity,” or in the “ Proceedings ” of the 
