# J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’ ```                                Measurement of Electrical Phenomena by Electrostatic Effects

(80) The quantities with which we have had to do have been
hitherto expressed in terms of the Electromagnetic System
of measurement, which is founded on the mechanical action
between currents. The electrostatic system of measurement
is founded on the mechanical action between electrified bodies
and is independent of, and incompatible with the electromagnetic
system, so that the units of the different kinds of quantity
have different values according to the system we adopt, and
to pass from one system to the other, a reduction of
all the quantities is required.

According to the electrostatic system, the repulsion
between two small bodies charged with quantities y<sub>1<\sub>y<sub>2<\sub> of
electricity is  y<sub>1y<sub>2<\sub>/r<sup>2<\sup>
where r is the distance between them
Let the relation of the two systems be such that one
electromagnetic unit of electricity contains v electrostatic
units then e<sub>1<\sub> = vy<sub>1<\sub> and e<sub>2<\sub> = vy<sub>2<\sub>
and k/4[pi] e<sub>1<\sub>e<sub2<\sub>/r<sup>2<\sup> = v<sup>2<\sup> e<sub>1<\sub>e<sub2<\sub>/r<sup>2<\sup> (45)
whence k the coefficient of "electric elasticity" in the medium
in which the experiments are made i.e. common air, is
related to v the number of electrostatic units in one
electromagnetic unit, by the equation
k = 4[pi]v<sup>2<\sup> (46)
The quantity v may be determined by experiment in several ways
According to the experiments of M.M. Kohlrausch 2 and Weber 1
v = 310.740.000 meters per second

```
images

## Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
IIIF (What's this?)
This is a link to the IIIF web URL for this item. You can drag and drop the IIIF image link into other compatible viewers

## Cite as

J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7