**Electrodynamic** A mmeters.

Now the electric force (P,Q,R) is the force acting on the electrons of the medium moving with velocity v; consequently by Faraday's **electrodynamic** law (P,Q,R) = (P',Q' - vc, R'+vb) where (P',Q',R') is the force that would act on electrons at rest, and (a,b,c) is the magnetic induction.

If v varies with respect to locality, or if there is a velocity of convection (p,q,r) variable with respect to direction and position, and analytical expression of the relation (ii) assumes a more complex form; we thus derive the most general equations of **electrodynamic** propagation for matter treated as continuous, anyhow distributed and moving in any manner.

These results constitute a far-reaching development of the modern or **electrodynamic** theory of the aether, of which the issue can hardly yet be foreseen.

Electromagnetic voltmeters may therefore be thermal, electromagnetic or **electrodynamic**. As a rule, electromagnetic voltmeters are only suitable for the measurement of relatively small potentials - o to 200 or 300 volts.

In 1846 Weber announced his famous hypothesis concerning the connexion of electrostatic and **electrodynamic** phenomena.

Ann., 1834, 31; 18 35, 34) among other results led him to the statement of the law by means of which the direction of the induced current can be predicted from the theory of Ampere, the rule being that the direction of the induced current is always such that its **electrodynamic** action tends to oppose the motion which produces it.

His work on the **electrodynamic** qualities of metals, thermo-electricity, and his contributions to galvanometry, were not less massive and profound.