Cua. VII., § 6.] 
adverted (32, dic.) The requirements of practice 
are magnificent experiments, such as no indivi- 
dual and no scientific Society would think of exe- 
euting for the illustration of theory. It is not in 
the least my purpose to transfer to these two gentle- 
men an exclusive merit which they need not be un- 
willing to share with other energetic and able com- 
petitors in the hard-run race of scientific applications. 
They occupy, however, perhaps the most marked and 
distinguished place, and the field is so wide and in- 
cludes so many minute details, that it requires all 
our resolution to fix our eyes steadily on the most 
considerable acquisitions—the nobler sheaves of so 
prolific a harvest. 
I shall connect, then, with the name of Mr 
Wheatstone, (1), the apparatus for determining the 
velocity of electrical conduction, (2), the electric tele- 
graph and clock,—with that of M. Jacobi, (3), elec- 
trodynamic machines, and (4), the electrotype. 
(855.) I. The apparatus used by Mr Wheatstone in 1834 
Mr Wheat- for measuring the velocity of the passage of the elec- 
=. on trical impulse through a good insulated conductor 
ie velo- : * ° 
city of elec- Such as a copper wire, deserves particular notice 
triccon- from its great ingenuity, and from its general appli- 
duction. cation to the measurement of short intervals of time. 
Let a copper conducting wire of half a mile long be 
so convoluted that the middle and the two ends of 
the wire may be brought near together, the whole 
being perfectly insulated. Let the wire be slightly in- 
terrupted at these three places, and the whole put 
| into connection at pleasure with an electric machine 
or battery. When contact is made, three sparks will 
| take place. Let the two end sparks be called A and C 
and the middle one B. As the three sparks take place 
close to each other, they can easily be seen at once 
{ reflected in a small plane mirror. Let now this small 
mirror be put in very rapid rotation round a hori- 
zontal axis so placed that the sparks (if they occur in 
the suitable part of the revolution) may be reflected 
together to the eye. Imagine the rotation to become 
immensely rapid:—in Mr Wheatstone’s apparatus 
the velocity reached 800 times in a second; conse- 
quently the mirror described 1° in gtaqth part of a 
second; i. ¢, in ggg'579 Of a second. But for 1° of 
rotation of a mirror the reflected image will describe 
an are of 2°. Supposing then that all the sparks occur 
at the same absolute instant of time, they will be 
seen in one line (supposing the points of the inter- 
rupted circuit in a line), but if either spark occur 
later than the others by only ggg%5ay Of a second, 
the mirror will have revolved so much in the interval 
as to displace the image of that spark relatively to 
the others by the very palpable angular amount of 
2°. In the copper wire half a mile long, the end 
sparks occurred simultaneously, whilst the middle 
spark occurred later by about one millionth of a 
second ; giving a velocity of transmission (according 
(854.) 
ELECTRICITY.—MM. WHEATSTONE AND JACOBI. 
985 
to Mr W.) of 288,000 miles a second, or somewhat 
greater than that of light.!_ The velocity in an iron 
telegraph-wire, ascertained lately in America with 
much greater accuracy, and by a different method, is 
only 16,000 English miles a second ; but doubts have 
been thrown upon the correct interpretation of these 
experiments. Those of M. Fizeau on the telegraphic 
lines of France give results more conformable to 
Mr Wheatstone’s, namely, about 70,000 English 
miles per second for iron, and 120,000 for copper 
wire. The duration of a spark drawn immediately 
from the battery is insensible, but in Mr Wheatstone’s 
experiment it lasted 375, of a second when trans- 
mitted by a copper wire half a mile long. 
II, Electric Telegraph and Clocks.—The idea of (856.) 
using the transmission of electricity to communicate Ele¢tric 
° r telegraph 
signals is so obvious as hardly to deserve the name _;;2 early 
of an invention, the prodigious velocity of common history. 
electricity in wires having been established by Watson 
before the middle of the last century. The earliest 
proposal of the kind appears in the Scots Magazine 
for February 1753, where a correspondent from Ren- 
frew, who signs himself C. M., proposes several kinds 
of telegraphs acting by the attractive power of electri- 
city, conveyed by a series of parallel wires correspond- 
ing in number to the letters of the alphabet, and in- 
sulated by supports of glass or jeweller’s cement at 
every twenty yards. Words are to be spelt by the 
electricity attracting letters, or by striking bells cor- 
responding to letters. One Lesage, in 1782, and even 
long before, proposed to convey twenty-four insulated 
wires in a subterranean tube, and to indicate the letters 
of the alphabet by means of the attraction of light 
bodies. In 1811 Sémmering suggested a similar ap- 
plication of voltaic electricity, chemical decomposition 
being the effect observed. Oersted first, and then Am- 
pére (1820) suggested the use of magnetic deflections 
for the same purpose, which is nothing else than the 
needle telegraph in general use in England ; but they 
contented themselves with the suggestion merely. 
MM. Gauss and Weber ‘communicated signals at 
Gottingen in 1833 or 1834 to a considerable dis- 
tance, and gave them the signification of letters. 
This was the first accomplishment of telegraphic com- 
munication by means of electricity, and it realized 
the fancy of Strada, quoted by Addison, of sym- 
pathetic magnets. It was, however, a mere appen- 
dage to a magnetic observatory, and its application 
and diffusion on a great scale seems to have required 
a distinct effort; for several years elapsed before we 
hear more of the telegraph. 
The year 1837 is the date of the realized electric (857. 
telegraph. We find three distinct claimants, of whose Telegraphs 
independent merits there is no reason whatever to Sy cco. 
doubt, though how much of the merit of all must be Steinheil, 
considered due to MM, Gauss and Weber, who first and Wheat- 
made the experiment, though they did not offer it *™* 
VOL. I. 
1 The numerical value is of course only a very rude estimation. 
61 
