Some experiments were made by putting bits of tinfoil between the plates whether the glasses were flat, and consequently whether the measures thus found were true. It seemed as if when the plates lay on each other the middle of the coatings could not want more than 002 or 004 of touching, but it did not appear that it wanted so much, and it seemed as if the outside did not want anything of touching, so that the above measures seem pretty just.

The diameter of the coating was 11·4.

The above coatings were taken off from these plates of glass, and coatings 6.254 in diameter put in their room, these with the small wax supports placed between them is called plate air 5.

N.B. 8 folds of the tinfoil used for these coatings was found to be 1 inch thinner than the same number of folds of that used for former coatings, so that this plate air is about 003 thicker than plate air 4.

The coatings were also taken from thin rosin and coatings 4-525 put in their room.

A plate of pure lac was also pressed out 125 thick, and the coatings used before for thin rosin put on, which were found at a medium 4.23 in diameter.

Two plates of dephlegmated* bees wax pressed out the year before were also coated.

N.B. The bees wax was heated very hot in dephlegmating, and melted with gentle heat when cast into plates.

518] Lac plate and 4th rosin compared with D + E + F; also thin wax with E+ F; also thick wax and plate air 5 with D.

Mon. Jan. 4 [1773]. Th. 51. N. 17.

[23 observations.]

By these exper. Lac plate contains 11⁄2 inc. el. more than D+E+F; 4th rosin from inc. more to 2 inc. less, by mean much the same as D+E+F; thin wax 4 inc. less than E+ F; thick wax 2 less than D ; plate air 5 more, and 1st made rosin more.

In the preceding experiments the plates of rosin &c. were exposed to the heat of the fire during trials, which seemed to cause an irregularity. To avoid that, in the following days' experiments the plates were laid on table at same distance from fire as the machine for some time before they were tried, and a screen was placed between all of them (except plate air) and fire while trying.

519] Lac and 4th rosin with D + E + F; also thin wax with D + E ; also thick wax, 2nd rosin and 1st made rosin and plate air 5 with F.

* [Art. 375.]

Tu. Jan. 5 [1773]. Th. 50. N. 17.

[22 observations.]

By these exper. 4th rosin contains from 2 inc. more to 21 less than D+E+F; by mean, the same as D + E + F.

Lac from 4 more to less, by mean 21 more than D + E + F.

Thin wax from 4 less to 1 less, by mean 2 less than D + E.

Thick wax 3 less than F.

2nd rosin, 12 less, plate air 5 1 more, and 1st made rosin same as F.

N.B. The 1st made rosin was made of the same proportion of rosin and bees wax as the others, but not of the same parcel: it is uncertain how much it was heated in making the mixture.

520] Breaking of electricity through thin plates of lac, exper. rosin and dephleg. bees wax.

Thin plates were pressed out of lac, experimental rosin and dephlegmated bees wax, very thin at one end and thicker at the other. The tinfoil was stript from one side of these plates but the other left on, and was fastened to a piece of glass with gum water, and a piece of tinfoil fastened to the under side of glass communicating with the other.

These plates were placed on [the] negative side of the machine with wire 8 bearing against bottom and a flat piece of brass at top on which wire was suffered to rest. The machine was electrified in usual degree, and the bit of brass shifted from thicker to thinner part, till the electricity broke through the plate and discharged the jars.

A piece of the plate with the tinfoil under it was then cut out of the size of the brass plate, as near as possible to the place where the electricity broke through, and the thickness of the plate found by weighing it and also the tinfoil after the plate was separated from it.

The thickness of the plates thus found was as follows †, the specific

* [See Art. 343. The inches of electricity are circular inches, and to reduce them to globular inches must be multiplied by 12.1, the diameter of globe, and divided by 18.8, the diameter of a circle which has the same charge. The computed power here is the square of the diameter divided by the thickness, and this must be multiplied by 8 to get the computed power as defined in Art. 311.]

64

[With the usual degree of electrification" Lane's electrometer discharged at 04 inch. See Art. 329. The electric strength of wax, rosin, and lac is therefore about three times that of air.]

521] The quantity of electricity in a Florence flask tried with and without a magazine.

The quant. el. in a Florence flask was tried by putting it on negative side, and some of the jars &c. on the other, the battery of 6 Florence flasks being used instead of the jars.

With the 1st, 2nd, 3rd jar with sliding plate 640 sep. neg. rather more than 1 diam.

4 jars white cyl.

1, 2, & 3 jars + 6 – 48

Sat. Jan.

sep. a little pos.
Do neg.

Th. 56. N. 19.

The same thing tried again in same manner

with the 4 jars and white cyl. sep. about 1 diam. pos.

With 4 jars and white cyl. with 1, 2, & 3 jar

neg.

pos.

sep. at 1 about 1 diam, but soon closed, sep. a good deal neg.

with 1, 2, & 3 jar + wh. cyl. + gr. cyl. 2 after a time sep. near 1 diam. With 4 jars and the 2 cyl., sep at 1st a good deal, after a time sep. about 1 diam.

A coating of tinfoil to a part of the Florence flask out of water. With 4 jars + wh. cyl. + gr. cyl. 2 sep. rather more than 1 diam.

The case was much the same whether wire was suffered to rest at bottom 2" or 3", or less than 1".

With 4 jars sep. a little neg., increased after a time to full 1 diam. By the 1st night's experiments the flask contains 12126 inc. el. by the 2nd

and by the 3rd

Without magazine by 2nd night it contained
The true quantity is supposed

11694

11495

13205 inc. el.

11700

522] Computed power of above flask.

The diameter of the flask at the surface of the water in tin pan on Saturday was 17; the height of that part above the bottom 5.1; the height of top of tinfoil coating above bottom 6.55; and the diameter of that place, 68; and the circumference at the widest part 13.

The weight of that part under water was 1..2.. 7*, and that of the part between that and the top of coating was 2..

4.

If the spheroid agdm does not differ much from a sphere, and ab does not differ much from ad, the surface afea is nearly equal to the circumference

(surface

power of the whole part below top of coating 6575, the specific gravity of the glass being supposed 2.68.

Therefore inc. el. by comp. pow. = 1.78.

523] As it appears from the above experiments that the Florence flask contains more electricity when it continues charged for a good while than when charged and discharged immediately, it was tried whether the white globes would do the same.

This was done by putting the globe 3 on positive side and the white cylinder and trial plate 6 on negative side, and first charging and discharging them in the common manner, and then discharging the magazine and charging it again, while the end c of the wire Cc rested on Bb, while the end C was prevented from resting on Aa by a silk string. When the magazine was charged, and had continued so for a little time, the end C was let down on Aa and the wire Cc immediately drawn up again so as to discharge the globe &c. The event was as follows,

1

By these experiments the globe contains 45 inc. el. or about less when electrified in the common way than when charged before the rest, which is as much as is contained in 1 inch in length of the uncoated part of the neck (the whole neck being 1 inches), so that supposing the experiment exact it seems as if the globe contained rather more elec[Troy weight.]

tricity when it continued charged a considerable time than when charged and discharged immediately*.

524] Diminution of shock by passing through different liquors†. Tried in November [1772].

The electricity was made to pass through 42 inches of a saturated solution of sea salt in a thermometer tube of a wide bore, and the two jars charged in such manner as that a slight shock should be felt in [the] elbows: it was then made to pass through rain water in a tube of a rather greater capacity, and the electricity made rather stronger. The wires were obliged to be placed within 18 of each other in order to feel the shock in the same degree. Therefore the electricity meets with more than 230 times the resistance in passing through rain water than salt.

The above jars were electrified till light paper cylinders began to separate, and the shock made to pass through a tube filled with rain water. The wires were obliged to be brought within 48 inches of each other in order that the shock should be just felt in the elbows.

When the same tube was filled with saturated solution of sea salt diluted with 29 its bulk of rain water, a much greater shock was felt when the wires were at 16 inches from each other.

Therefore electricity meets with much more than 34 times the resistance from rain water than from a saturated solution of sea salt with 29 of rain water.

When the same tube was filled with kitchen salt in 1000 of rain water, the wires must be brought within 4·4 inches; with pump water within 2 inches, and with spirit of wine almost close; therefore the resistance of

Pump water

(43 is less than that of rain water.

S. salt in 1000 of rain water) 19

Mon. Nov. 16 [1772] with straw electrometer. With sea water a shock was felt when the wires were 19 inches distant; with rain water when they were at about 19 inches distant. Therefore resistance of sea water is about 100 times less than that of rain water.

525] Exper. Whether force with which two bodies repel is as square of redundant fluid in them.

* [These phenomena are connected with the 'residual charge.' A careful investigation of them has been made by Dr Hopkinson, Phil. Trans., Vol. 167 (1877), p. 599.] + [This is the first experiment on electric resistance.]

+

+

[Arts. 386, 563, 567.]