Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/427
misled by the analogy between electricity and heat, entertained an opinion of this kind, and was thus, by means of an erroneous opinion, led to employ the equations of Fourier to express the true laws of conduction of electricity through a long wire, long before the real reason of the appropriateness of these equations had been suspected.
Mechanical Illustration of the Properties of a Dielectric.
334.] Five tubes of equal sectional area and are arranged in circuit as in the figure. and are vertical and equal, and is horizontal.
The lower halves of are filled with mercury, their upper halves and the horizontal tube are filled with water.
A tube with a stopcock connects the lower part of and with that of and and a piston is made to slide in the horizontal tube.
Let us begin by supposing that the level of the mercury in the four tubes is the same, and that it is indicated by that the piston is at and that the stopcock is shut.
Now let the piston be moved from to a distance Then, since the sections of all the tubes are equal, the level of the mercury in and will rise a distance or to and and the mercury in and will sink an equal distance or to and
The difference of pressure on the two sides of the piston will be represented by
This arrangement may serve to represent the state of a dielectric acted on by an electromotive forceThe excess of water in the tube may be taken to represent a positive charge of electricity on one side of the dielectric, and the excess of mercury in the tube may represent the negative charge on the other side. The excess of pressure in the tube on the side of the piston next will then represent the excess of potential on the positive side of the dielectric.