Velocity of the Galvanic Current in Telegraph Wires. 81 
by which Mr. Fizeau had previously measured the velocity of 
light between Paris and Montmartre. A wheel, whose circum- 
ference consisted of 36 divisions, alternately of wood and _plati- 
num, revolved in contact with the edges of insulated plates of 
platinum, each pair of plates forming a distinct circuit-breaker. 
The driving machinery of the wheel was connected with an ap- 
| paratus for measuring the velocity of rotation ; and the effect of 
‘ different velocities and different circuits was observed upon the 
1 galvanometer.* 
| The velocity of the current, which they deduced from these 
experiments, was, as I have already mentioned, 63,200 miles per 
second in iron wire 4 millimeters in diameter, and 110,000 in 
copper wire, 24 millimeters in thickness. 
They inferred, moreover, 
That the two electricities were propagated with the same ve- 
locity.t+  - 
_ That the tension of the electricity has no influence on the ve- 
locity. 
That the velocity does not vary with the section of the con- 
ducting material, but only with its nature, and then not in the 
ratio of the conductive power xs od seameaees 
That the discontinuous currents “experience a diffusion, in 
consequence of which they occupy a space greater at the point 
of arrival than at the point of departure.” ; & 
This latter result is in direct opposition to the experiments of 
Walker with the chemical telegraph, by which, using a method 
of determination indefinitely less complicated he found the length 
of duration of the electric currents to be less at the more remote 
Station. According to Fizeau,t the electropea 3s propagated 
more rapidly than the electrotome ;—according to Walker,§ less 
rapidly. Fizeau used a circuit 374 miles in length, and Walker 
one of 250 miles of wire, whose two extremities communicated 
with the ground about 190 miles apart. eg 
The Coast Survey experiments of Feb. 4, on the line between 
Washington and St. Louis, 1045 miles distant by the wires, and 
742 in a geodetic line, lead me to suppose, as already stated, that 
the velocity of the two signals is the same, and (other things be- 
ing equal) the pauses would be of equal length, as recorded at all 
the stations on the circuit. I am inclined to attribute Walker's 
results to the more intense chemical action of the current on the 
prepared paper at the signal stations,—and while entertaining the 
most profound admiration for the remarkable ingenuity and care 
Shown by Fizeau, I cannot consider his method as capable of 
_ Slving results worthy of implicit reliance. 
+ Faraday, Res., i, $1833. 
§ Ast. Journ., i, p. 106. 
5 
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