THE

ELECTRICAL RESEARCHES

OF THE

HONOURABLE HENRY CAVENDISH, F.R.S.

INTRODUCTION *.

So little is known of the details of the life of Henry Cavendish, and so fully have the few known facts been given in the Life of Cavendish by Dr George Wilsont, that it is unnecessary here to repeat them except in so far as they bear on the history of his electrical researches.

He was born at Nice on the 10th October, 1731, he became a Fellow of the Royal Society in 1760, and was an active member of that body during the rest of his life. He died at Clapham on the 24th February, 1810.

His father was Lord Charles Cavendish, third son of William, second Duke of Devonshire, who married Lady Anne Grey, fourth daughter of the Duke of Kent. Henry was their eldest son. He had one brother, Frederick, who died 23rd February, 1812.

Of Lord Charles Cavendish we have the following notice by Dr Franklint. After describing an experiment of his on the passage of electricity through glass when heated to 400° F., he

says,

"It were to be wished that this noble philosopher would communi"cate more of his experiments to the world, as he makes many, and "with great accuracy.”

By the Editor.

+ Published in 1851 as the first volume of the Works of the Cavendish Society. Franklin's Works, edited by Jared Sparks, Boston, 1856, Vol. v, p. 383. See

also Note 26 at the end of this book.

Lord Charles Cavendish has also recorded a very accurate series of observations* on the depression of mercury in glass tubes, and these have furnished the basis not only for the correction of the reading of barometers, &c., but for the verification of the theory of capillary action by Young, Laplace, Poisson and Ivory.

I think it right to notice the scientific work of Lord Charles Cavendish, because Henry seems to have been living with him during the whole period of his electrical researches. Some of the jottings of his electrical calculations are on torn backs of letters, one of which is addressed,

These calculations relate to the equivalent values of his trial plates when drawn out to different numbers of divisions. There is no date nor any part of the original letter.

The memoranda of some experiments similar to those in Art. 588, on the time of discharge of electricity through different bodies, are on the back of the usual Notice of the election of the Council and Officers of the Royal Society on the Thirtieth of November, 1774 (being St Andrew's Day) at Ten o'Clock in the Forenoon at the House of the Royal Society in Crane Court, Fleet Street. The address on the back of this letter is

Το

The Hon Henry Cavendish

Gr Marlborough Street.

Dr Thomas Thomson, who was acquainted with Cavendish, says in his interesting sketch of himt,

"During his father's life-time he was kept in rather narrow circum"stances, being allowed an annuity of £500 only, while his apartments

* Phil. Trans., 1776, p. 382.

+ History of Chemistry, Vol. 1, p. 336, quoted in Wilson's Life of Cavendish, p. 159.

were a set of stables, fitted up for his accommodation. It was during "this period that he acquired those habits of economy and those singular "oddities of character which he exhibited ever after in so striking a "manner."

The whole of the electric researches of which we are to give an account were made before the death of Lord Charles Cavendish, which took place in 1783. We must therefore suppose that they were made in Great Marlborough Street, and probably in the set of stables mentioned by Dr Thomson. He speaks of a "fore room and a back room" in Art. 469, and in Art. 335 he compares the size of the room in which he worked to that of a sphere 16 feet in diameter. The dimensions of his laboratory are of some importance in determining the electric capacity of bodies hung up in it, and by the foot-note to Art. 335 it would appear that the room was probably 14 feet high, which is somewhat lofty for "a set of stables," but I believe not much more than the height of some of the rooms in the dwelling-houses in Great Marlborough Street.

Let us then suppose that we have been admitted by Cavendish into his laboratory in Great Marlborough Street, as it was arranged for his electrical experiments in 1773, and let us make the best of an opportunity rarely, if ever, accorded to any scientific man of his own time, and examine the apparatus by which the electric fluid, instead of startling us with the brilliant phenomena, new instances of which were then every day being discovered, was made to submit itself, like everything else which entered that house, to be measured.

The largest piece of apparatus was the "machine for trying simple bodies" of which we have a description and sketch in Art. 241, and plans at Arts. 265 and 273. The framework of the machine is not represented in these figures.

We learn, however, from Dr Davy*, that

"Cavendish seemed to have in view, in construction, efficiency "merely, without attention to appearance. "used, excepting when required. Fir-wood

66

Hard woods were never (common deal) was that

commonly employed."

* Wilson's Life, p. 178.

The bodies to be "tried" and the wires and vials for trying them were either supported on glass rods as shown in the sketch at Art. 239, or else hung by silk strings from a horizontal bar 7 feet 3 inches from the floor as mentioned in Art. 466. The electrical connexions were made and broken at the proper times by means of silk strings passing over pullies attached to the horizontal bar.

One of the bodies, the charges of which Cavendish compared by means of this apparatus, was a globe 121 inches in diameter covered with tinfoil. This globe has historical interest as it was not only the standard of capacity with which Cavendish compared that of all other bodies, but it formed part of the apparatus by which he established that the electric repulsion varies inversely as the square of the distance.

There was also a set of circles of tin plate, one of 36 inches diameter, one of 18.5 and two of 93; and also square and oblong tin plates, and squared pieces of stone and slate, and a collection of cylinders and wires of different sizes.

There was another "machine," represented, with its framework, in Fig. 20, Art. 295, "for trying Leyden vials."

The "Leyden vials" were most of them flat plates of glass with circular coatings of tinfoil, one on each side. They were made in sets of three, any one of each set being nearly equal in capacity to the three of the former set taken together. Cavendish had thus a complete set of condensers of known capacity by means of which he measured the capacity of every piece of his apparatus, from the little wire which he used to connect his coated plates, and which he found to contain 28 "inches of electricity," up to his battery of 49 jars, which contained 321000 "inches of electricity "*.

These "inches of electricity" can be directly compared with our modern measurements of electrostatic capacity. Indeed the only difference is that Cavendish's " inches of electricity" express the diameter of the sphere of equivalent capacity, while the modern measurements express the capacity by stating the radius of the same sphere in centimetres.

* About half a microfarad.