128 
SCIENCE. 
the age of thirty-thtee, he moved to Princeton College. After a 
year's break in his work, caused by the preparation of his course 
of lectures for the college, he is again at original research, and 
continues his contributions to electrical discoveries till 1842. Thus, 
during fourteen years, between the ages of twenty-eight and forty- 
three, he was a constant and fertile woiker. 
As with many oiher men of originality, Henry’s first essays were 
in the direction of improving the means of illustrating well-estab- 
lished scientific facts and principles. His first paper of October, 
1827, is interesting because it was his first. In it he improves on 
the usual apparatus which had been used by Ampere and others 
to show electro-dynamic actions, by employing several turns of in- 
sulated wire, instead of one, as had previously been the practice. 
Thus, for example, to show the directive action of the earth's 
magnetism on a freely-moving closed circuit, Henry covered cop- 
per wire with silk, and then made out of it a ring about twenty 
inches in diameter, formed of several turns of the wire. The ex- 
tremities of this wire were soldered to zinc and copper plates. The 
coil was then suspended by silk filaments. On plunging the metal 
plates into a glass of dilute acid the ring rotated around its point 
of suspension till its plane took a permanent position at right 
angles to the magnetic meridian. By a similar arrangement of 
two concentric coils, one suspended within the other, he neatly 
showed the mutual actions of voltaic currents flowing in the same 
or opposite directions, which facts are the founda ions of Ampere’s 
celebrated law. We now reach a period when Henry appears as 
a discoverer, and truly one of no mean order. As I remember his 
narration to me in the year 1859, it was as follows : He said that 
one evening he was sitting in his study in Albany with a friend, 
when, after a few moments of reverie, he arose and exclaimed, 
“ Tomorrow I shall make a capital experiment ! " For several 
months he had been brooding over Ampere's electro-dynamic 
theory of magnetism, and he was then deeply interested in the 
phenomenaof the development of magnetism in soft iron, as shown 
in the experiments of Arago and Sturgeon. At the moment he had 
arisen from his chair it had occurred to him that the requirements 
of the theory of Ampere were not fulfilled in the electro-magnets 
of Arago and of Sturgeon, but that he could get those conditions 
which the theory required by covering the developing wire with a 
non-conductor, like silk, and then wrapping it closely around the 
soft iron bar in several layers ; for the successive layers of wire 
coiling first in one direction and then in the other would tend to 
produce a resultant action of the current at right angles to the 
axis of the bar ; and furthermore, the gyeat number of convolutions 
thus obtained would act on a greater number of molecules of the 
bar, and therefoie exalt its magnetism. “ When this conception,” 
said Henry, “ came into my brain, I was so pleased with it that I 
could not help rising to my feet and giving it my hearty approba- 
tion.” Henry did go to work next day, and to his great delight 
and encouragement discoveries of the highest interest and import- 
ance revealed themselves to him week alter week. When he had 
finished his newly conceived magnet he found that it supported 
several times more weight than did Sturgeon's magnet of equal 
size and weight. This was his first original discovery. 
I will now give, as far as possible, Henry's own words in narrat- 
ing the subsequent investigations of these very interesting phe- 
nomena: "The maximum effect, however, with this arrangement 
and a single battery was not yet obtained. After a certain length 
of wire had been coiled upon the iron the power diminished with a 
further increase of the number of turns. This was due to the in- 
creased resistance which the larger wire offered to the conduction 
of electricity. Two methods of improvement, therefore, suggested 
themselves. The first consisted, not in increasing the length of 
coil, but in using a number of separate coils on the same piece of 
iron. By this arrangement the resistance to the conduction of the 
electricity was diminished and a greater quantity made to circulate 
around the iron from the same battery. The second method of 
producing a similar result consisted in increasing the number of 
elements of the battery, or, in other words, the projectile force of 
the electricity, which enabled it to pass through an increased num- 
ber of turns of wire, and thus, by increasing the length of the wire, 
to develop the maximum power of the iron. To test these princi- 
ples on a larger scale an experimental magnet was constructed. 
In this a number of compound helices were placed on the same 
bar, their ends left projecting, and so numbered that they could 
be all united into one long helix, or variously combined in sets of 
lesser length. From a series of experiments with this and other 
magnets it was proved that, in order to produce the greatest 
amount of magnetism from 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, making many turns around the 
iron, the length of wire and consequently the number of turns 
being commensurate with the projectile power of the battery. In 
describing the results of my experiments the terms intensity and 
quantity magnets were introduced to avoid circumlocution, and 
were intended to be used merely in a technical sense. By the in- 
tensity magnet I designated a piece of soft iron so surrounded 
with wire that its magnetic power could be called into operation 
by an intensity battery ; and by a quantity magnet a piece of iron 
so surrounded by a number ot separate coils that its magnetism 
could be fully developed by a quantity battery. ”1 was,” said 
Henry, “ the first to point out this connection of the two kinds 
of the battery with the two forms of the magnet, in my paper in 
Silliman’s Journal, January, 1831, and clearly to state that when 
magnetism was to be developed by means of a compound battery 
one large coil was to be employed, and when the maximum effect 
was to be produced by a single battery a number of strands were 
to be used.” 
We will now return to Henry's study of the properties of his 
intensity magnet. This magnet was formed of a piece of iron 
one-fourth of an inch in diameter, bent in the U form and wound 
with eight feet of insulated wire. His batteries were two, — one 
formed of a single element with a zinc plate four inches by seven, 
surrounded by copper and immersed in dilute acid ; the other, a 
Cruikshank's battery, or trough, with twenty-five double plates. 
The plates of this battery were joined in series, and altogether had 
exactly the same surface of zinc as that in the single-cell battery. 
The magnet was now connected directly to the single cell. The 
magnet held up seventy-two ounces. Then five hundred and 
thirty feet of number 18 copper wire led the current from the cell 
to the magnet; it now supported only two ounces. Five hundred 
and thirty feet more of the wire were introduced into the circuit, 
and then the magnet held but one ounce. In these facts Henry 
faced the same results as confronted Barlow five years before, and 
caused Barlow then to say: "In a very early stage of electro- 
magnetic experiments it had been suggested [by Laplace, Ampere 
and others] that an instantaneous telegraph might be established 
by means of conducting wires and compasses, but I found such a 
sensible diminution with only two hundred feet of wire, as at once 
to convince me of the impracticability of the scheme " ; and such, 
at that day, seemed to be the common opinion of men of science. 
But this opinion is presently to be shown by Henry to be ill-found- 
ed, by reason of the ignorance of the relations which have of 
necessity to exist between the kind of battery and the kind of 
magnet in order to produce electro-magnetic action at a distance- 
relations which Henry was the first to discover. This accomplish- 
ment justly entitles him to be regarded as a man of genius and a 
discoverer of no mean order. This discovery will always remain 
the one important fact that was to be known, to be understood, 
and to be applied, before it was possible to have constructed any 
form of electro-magnetic telegraph. Let us see how Henry made 
this discovery. 
After ending the experiments with the one-cell battery and 
reaching resulis which seemed to confirm the opinion of Barlow 
as to the “ impracticability of the scheme " of an electro-magnetic 
telegraph, Henry attached his magnet to the second battery 
formed of twenty-five cells, arranged in series. The current from 
this battery was sent to the magnet through 1060 feet of the same 
wire as had been used in the experiments with the first battery of 
one cell. The magnet now lifted eight ounces. It had held up 
only one ounce, when with the same length of interposed wire the 
battery of one cell was used. He now attached his electro-magnet 
directly to the poles of the 25-cell battery, when, to his astonishment, 
it only held seven ounces. The same magnet it will be remem- 
bered, when attached to the one-cell battery, supported seventy- 
two ounces. Here were facts of the highest significance, and 
Henry was not slow to seize them in all their bearings. Referring 
to these experiments, he said in 1857 : “ These steps in the ad- 
vance of electro-magnetism, though small, were such as to interest 
and astonish the scientific world. These developments were con- 
sidered at the time of much importance in a scientific point ol 
view, and they subsequently furnished the means by which 
magneto-electricity, the phenomena of dia-magnetism, and the 
magnetic effects in polarized light were discovered. They 
gave rise to the various forms of electro-magnetic ma- 
chines which have exercised the ingenuity of inventors in 
every part of the world, and were of immediate applicability in 
the introduction of the magnet to telegraphic purposes. Neither 
the electro-magnet of Sturgeon nor any electro-magnet ever made 
previous to my investigations was applicable to transmitting power 
to a distance.” 
Not satisfied with the mere statement that his discovery was 
" directly applicable to Mr. Barlow's project of forming an electro- 
magnetic telegraph,” he actually constructed one, some time dur- 
ing the year 1831, around one of the upper rooms of the Albany 
Academy. It was more than a mile in length, and made signals 
by sounding a bell. This was the first electro-magnetic telegraph 
which had worked through so great a length of wire. It was the 
first “sounding ” electro-magnetic telegraph. The relative parts 
played by Henry and Morse are described in Henry's “ State- 
ment” published by the Smithsonian in 1857. "The principles ,” 
says Henry, “ I had developed were applied by Dr. Gale to renda 
Morse's machine effective at a distance." This statement seems to 
be as direct, as clear, as truthful, and as comprehensive as one 
can desire. I will take the liberty of remarking that had Henry 
taken out a patent in which he claimed as his invention an electro- 
magnet formed of two or more layers of insulated wire, Morse's 
patent would not have been so valuable. Remember, I speak not 
ot the merit of the inveniion, but of the merit of the patent ; for 
the invention, so far as Morse is concerned, would have remained 
the same, because one essential part of a Morse telegraph is 
Henry's intensity magnet, and certainly Morse never invented 
that. 
If Ohm's law had been known to Henry, with all of its conse- 
quences, when applied to his discovery of the exaltation ot the 
electro-magnetism of iron, in connection with his discovery of the 
