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370] CHARGE OF GLASS AT DIFFERENT TEMPERATURES.

181

It was heated by a lamp placed under FGMN, and its charge was frequently tried while heating by comparing with a sliding coated plate placed on the other end of the wooden bars.

When it was sufficiently heated, the lamp was taken away, and the charge frequently tried in the same manner while cooling, a thermometer being dipt every now and then into the mercury in FGMN to find its heat.

367] As it was apprehended that the electricity might spread further on the surface of the glass while hot than while cold, a paper coating DBbd was fastened on the tube, so that as the outside coating was made to extend as far as Dd, that is three or four inches above the mercury in FGMN, where the tube was very little heated, and as the inside coating reached still higher, that is to the bottom of the upper ball, no sensible error could proceed from thence.

The use of the upper ball was to prevent the mercury within the tube from overflowing when hot.

368] By a mean between the experiments made while the ball was heating and while cooling, its charge answering to the different degrees of heat was as follows.

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369] At 295° the electricity passed through the glass pretty freely, but at 305° much faster. It appears, therefore, that the charge of glass is considerably greater when heated to such a degree as to suffer the electricity to pass through than when cold, but that its charge does not begin to be sensibly increased till it is heated to a considerable degree*.

370] On the charges of plates of several different sorts of glass, and also of plates of some other substances which do not conduct electricity, charged in the manner of Leyden vials.

* [Note 26.]

The result of the experiments I made on this subject is contained in the two following tables:

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372] The coatings of all these plates were circular.

In computing the charge of the glass plates, the diameter of the coating was corrected on account of the spreading of the electricity as in the fourth column, the electricity being supposed to spread 07 of an inch if the thickness is 21 and 09 if the thickness is 08, and so on in proportion in other thicknesses. But no correction is made in computing the charges of the other plates, as I was uncertain how much to allow.

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376]

BEES WAX, ROSIN, AND SHELLAC.

183

373] The method I used in making all the plates of the second table was this. I first cast a round plate of the substance, three or four times as thick as I intended it should be, and rather thinner near the edges than in the middle, taking care to cast it as free from air bubbles as I could.

I then heated it between two thick flat plates of brass, till it was become soft, and then pressed it out to the proper thickness by squeezing the plates together with screws. In order to prevent its sticking to the brass plates, I put a piece of thin tinfoil between it and each plate, and I found the tinfoil did not stick to it so fast but what I could get it off without any danger of damaging them.

374] The heat necessary to melt shell lac is so great as to make it froth and boil; which makes it impossible to cast a plate of it free from air bubbles. The plate mentioned in the preceding table was as free from them as I could make it. It contained, however, a great quantity of minute bubbles, but no large ones.

375] Bees wax melts with a heat of about 145o. If it is then heated to a degree rather greater than that of boiling water, it froths very much, and seems to lose a good deal of watery matter, and if it is kept at this heat till it has ceased frothing, it will then bear being heated to a much higher degree without frothing or boiling. Bees wax thus prepared I call dephlegmated.

In order that the plates of dephlegmated bees wax should all be equally so, I dephlegmated some bees wax with a pretty considerable heat, and suffered it to cool and harden, and out of this lump I made all three plates, taking care in casting them not to heat them more than necessary.

I used the same precautions also in casting the plates of a mixture of rosin and bees wax, the proportion of the rosin to the bees wax was forgot to be set down.

What are called in the table the 4th and 5th plate of rosin and bees wax are in reality the same plate as the 3rd, only with a smaller coating.

376] It appears from these experiments, first, that there is a very sensible difference in the charge of plates of the same

* [Art. 514.]

dimensions according to the different sort of glass they consist of, the charge of the plates O and Q, which consisted of the greenish foreign plate glass mentioned in [Art. 301] being the greatest in proportion to their computed charge of any, next to them the crown glass, and the flint glass being the least of all.

Secondly. The charge of the Lac plate is much less in proportion to its computed charge than that of any glass plate, and that of a plate of bees wax, or of the mixture of rosin and bees wax still less.

But it must be observed that there is a very considerable difference between the three different plates of dephlegmated bees wax in that respect. The same thing, too, obtains in the mixture of rosin and bees wax*.

377] As the proportion of the real charge to the computed is greater in the thick plates than the thin ones, one might be inclined to think that this was owing to the electricity being not spread uniformly. But as the difference seems to be greater than could well proceed from that cause, I am inclined to think that it must have been partly owing to some difference in the nature of the plates. Perhaps it may have been owing to some of the plates having been less heated, and consequently having suffered a greater degree of compression in pressing out than the others.

378] The piece of ground crown glass mentioned in the first of the foregoing tables was made out of a piece of crown glass about † of an inch thick, and ground down to the thickness mentioned in the table, care being taken by the workman to take away as much from one side as the other, so that the plate consisted only of the middle part of the glass.

My reason for making it was that as there appears to be a considerable difference in the charge of different sorts of glass, it was suspected that there might possibly be a difference between the inside of the piece and the outside, and if there had, it would have affected the justness of the experiments with the ten pieces of glass ground out of the same piece.

But by comparing the charges of the plates of crown glass with those of the two other pieces of crown glass in the table, [Note 27.]

There are pieces of that thickness sometimes blown for the use of the Opticians.

380]

COMPOUND PLATE.

185

there does not seem to be any difference which can be depended on with certainty.

The experiment indeed would have been more satisfactory if the piece of ground glass and the pieces with which it was compared had been all made out of the same pot. But as it would have been difficult procuring such pieces, and as I have found very little difference in the specific gravity of different pieces of crown glass, and as I am informed it is all made at the same glass house, I did not take that precaution.

Fig. 29.

A

B

C

D

379] Let two or more flat plates of different non-conducting substances, as AabB, Bbc C and Ccd D, (Fig. 29) be placed close together and coated in the manner of a single plate with the coatings Ee and Ff. Let the charge of the plate Aab B, supposing it placed by itself and coated in the usual manner, be equal to that of a plate of glass whose thickness is A and whose coatings are of the same size as those of AabB.

In like manner let the charge of Bbc C be equal to that of a plate of the same glass whose thickness is equal to B, and let that of CcdD equal that of one whose thickness is C.

E

a

Then whichever of the three ways of accounting for the excess of the real charge of glass plates above the computed we prefer, it is a necessary consequence of our theory that the charge of this compound plate AadD should be equal to that of a single plate of glass whose thickness equals A+B+C, and whose coatings are of the same size as Ee and Ff.

380] In like manner if two or more plates of the same kind of glass are placed together and coated as above, the charge of this compound plate should be equal to that of a single plate of the same glass whose thickness is equal to that of all the plates together. This appears from the following experiments to be the case, for

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