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Besides these he had “double” plates of very small capacity made of two plates of glass stuck together, and also other plates of wax and rosin, the inductive capacity of these substances being, as he had already found, less than that of glass; and jars of larger capacity, ranging up to his great battery of 49 jars, whose capacity was 321000 “inches of electricity." In estimating the capacity of his battery, he used the method of repeated touching with a body of small capacity. (Arts. 412, 441, 582.) This method is the same as that used by MM. Weber and Kohlrausch in their classical investigation of the ratio of the electric units*
Thus the method of experimental research which Cavendish adhered to was the comparison of capacities, and the formation of a graduated series of condensers, such as is now recognised as the most important apparatus in electrostatic measurements.
We have next to consider the steps by which he established the accuracy of his theory, and the discoveries he made respecting the electrical properties of different substances.
Cavendish himself, in his description of his experiments, has shown us the order in which he wishes us to consider them. The first experimentt is that of the globe within two hemispheres, from which he proves that the electric force varies inversely as the square of the distance, or at least cannot differ from that ratio by more than a fiftieth part. The degree of accuracy of all the experiments was limited by the sensitiveness of the pith ball electrometer which he used. Bennett's gold leaf electrometer, which is much more sensitive, was not introduced till 1787, but in repeating the experiment we can now use Thomson's Quadrant electrometer, and thereby detect a deviation from the law of the inverse square not exceeding one in 72000. See Note 19.
The second experiment, Art. 235, is a repetition of the first with bodies of different shape.
The third experiment, Art. 265, shows that in comparing the charges of bodies, the place where the connecting wire touches the
Elektrodynamische Maasbestimmungen, Abh. iv. p. 235. + Arts. 217 to 235.
body, and the form of the connecting wire itself, are matters of indifference.
The fourth experiment, Art. 269, shows that the charges of bodies of the same shape and size, but of different substances, are equal.
The fifth, Art. 273, compares the charge of a large circle with that of two of half the diameter. According to the theory the charge of the large circle should be equal to that of the two small ones if they are at a great distance from each other, and equal to twice that of the small ones if they are close together. Cavendish tried them at three different distances and compared the results with his calculations.
The sixth experiment, Art. 279, compares one long wire with two of half the length and half the diameter, placed at different distances.
The seventh, Art. 281, compares the charges of a globe, a circle, a square, an oblong and three different cylinders, and the eighth, Art. 288, shows that the charge of the middle plate of three parallel plates is small compared with that of the two outer ones.
Cavendish next describes his experiments for comparison of the charges of coated plates of glass and other substances, but begins by examining the sources of error in measurements of this kind.
The first of these which he investigates is the spreading of electricity on the surface of the plates beyond the coatings of tinfoil. He distinguishes two kinds of this spreading, one a gradual creeping of the electricity over the surface of the glass, Art. 300, and the other instantaneous, Art. 307.
He attempted to check the first kind by varnishing the glass plates and by enclosing their edges in a thick frame of cement, but he found very little advantage in this method, and finally adopted the plan of performing all the operations of the experiment as quickly as possible, so as to allow very little time for the gradual spreading of the electricity.
He next investigated the instantaneous spreading of electricity on the glass near the edge of the coating. He noticed that at the instant of charging the plate in the dark, a faint light could be seen all round the edges. He also observed that after charging
and discharging a coated plate of glass many times without cleaning it, a narrow fringed ring of dirt could be traced all round the coating, the space between this ring and the coating being clean, and in general about to inch broad.
He also observed that the flash of light was stronger the first or second times of charging a plate than afterwards.
To determine how much the capacity of a coated plate was increased by this spreading of the electricity, he compared the capacity of a plate with a circular coating with that of the same plate with a new coating of nearly the same area, but cut into strips, so that its perimeter was very much greater than that of the circular coating.
In this way he found that if we suppose a strip of uniform breadth added to the coating all round its boundary, the capacity of this coating, supposing the electricity not to spread, will be equal to that of the actual coating as increased by the spreading of the electricity. The most probable breadth of this strip he found to be 0:07 inch for thick glass and 0:09 for thin.
When this correction was applied to the areas of the coatings of the different coated plates, the computed charges of plates made of the same kind of glass were found to be very nearly in the same ratio as their observed charges.
But the observed charges of coated plates were found to be always several times greater than the charges computed from their thickness and the area of their coatings, the ratio of the observed charge to the computed charge being for plate glass about 8-2, for crown glass about 8:5, for shellac about 4:47, and for bees' wax about 3-5. Thus Cavendish not only anticipated Faraday's discovery of the Specific Inductive Capacity of different substances, but measured its numerical value in several substances.
The values of the specific inductive capacity of various substances as determined by different modern observers are compared with those found by Cavendish in the table in Note 27.
To make it certain, however, that the difference between the observed and calculated capacities of coated plates really arose from the nature of the plate and not from some error in the theory, Cavendish determined the capacity of a "plate of air,” that is to say a condenser consisting of two.circles of tinfoil on glass with air between them. The capacity of a plate of air was found to be much less than that of a plate of glass or of wax of the same dimensions, but it seemed to be about 1 in excess of the calculated value. This discrepancy will be discussed in Note 17.
These may be considered the principal results of the investigations with coated plates, but the following list of collateral experimental researches will show how thoroughly Cavendish went to work.
A question of fundamental importance in the theory of dielectrics is whether the electric induction is strictly proportional to the electromotive force which produces it, or in other words, is the capacity of a condenser made of glass or any other dielectric the same for high and for low potentials ?
The form in which Cavendish stated this question was as follows* :-"Whether the charge of a coated plate bears the same proportion to that of a simple conductor, whether the electrification is strong or weak.”
Cavendish, who explained the fact that the capacity of a glass plate is greater than that of an air plate, by supposing that the electricity is free to move within certain portions of the glass, supposed that when the plate was more strongly electrified the electricity would be able to penetrate further into the glass, and that therefore its charge would be greater in proportion to that of a simple conductor or a plate of air the stronger the degree of electrification.
But according to the experiments he made to answer this questiont a coated plate and a simple conductor whose charges were equal for the usual degree of electrification remained sensibly equal for higher and lower degrees, and if, as appeared probable from the experiments on the spreading of electricity at the edge of the coating, this spreading extended further for high degrees of electrification than for low, it would be necessary to admit that the charge of a glass plate became less in proportion to that of a simple conductor as the degree of electrification increased. Cavendish, however, concluded that the experiments were hardly accurate
enough to warrant the deduction from them of so improbable a conclusion.
He also found that the result of the comparison of a coated plate and a simple conductor was the same whether they were charged positively or negatively.
He tried whether the capacity of a plate of rosin altered with the temperature, but he could not find that it did* In glass he found that the capacity increased as the temperature rose, but the most decided increase did not occur till the glass began to conduct somewhat freely. Cavendish therefore does not consider the experiment quite decisivet.
He found that the apparent capacity of a Florence flask I was greater when it continued charged a good while than when it was charged and discharged immediately, and he found that the same was the case with a coated globe of glass. This phenomenon, which Faraday called “electric absorption,” has recently been carefully studied in different kinds of glass by Dr Hopkinsong. It is connected with the long-known phenomenon of the “residual charge,” and the existence of such phenomena in many dielectrics renders it difficult to obtain consistent values of their inductive capacities; for the more rapidly the charging and discharging is effected the lower is the apparent value of the capacity. It is for this reason that condensers of glass cannot be used as standards of capacity when accurate measurements are desired.
Franklin had shown || that the charge of a glass condenser resides in the glass and not in the coatings, for when the coatings were removed they were found to be without charge, and when new coatings were put in their place the condenser thus reconstructed was found to be charged.
Cavendish tried whether this was the case with a chargea plate of air, by lifting one of the electrodes and changing the air between them and then replacing the electrode. He found that the charge was not altered during these operations, and concluded that the charge resides, not in the air, but in the metal plates.
* Art. 523.
+ Art. 366. § Phil. Trans. 1877, p. 599. il Franklin's Works, ed. Sparks, Vol. v., p. 201.