In dry weather the electric potential in the atmosphere is normally positive relative to the earth, and increases with the height.
Varley, who proposed to make use of it in a telegraphic receiving instrument.4 In Dolbear's instrument one plate of a condenser was a flexible diaphragm, connected with the telephone line in such a way that the varying electric potential produced by the action of the transmitting telephone caused an increased or diminished charge in the condenser.
An exactly similar expression holds good in hydrokinetics, provided that for the electric potential we substitute velocity potential, and for the electric force the velocity of the liquid.
A voltmeter isan instrument for measuring difference of electric potential in terms of the unit called a volt.
Neglecting the external heat-loss, and the variation of the thermal and electric conductivities k and k', we obtain, as before, for the difference of temperature between the centre and ends, the equation O, Tho z Go = C 2 R1/8qk = ECl/8qk = E 2 k'/8k, (11) where E is the difference of electric potential between the ends.
