THE EXPERIMENTS OF STEPHEN GRAY

THE EXPERIMENTS OF STEPHEN GRAY

In 1729 Stephen Gray (died in 1736), an eccentric and irascible

old pensioner of the Charter House in London, undertook some

investigations along lines similar to those of Hauksbee. While

experimenting with a glass tube for producing electricity, as

Hauksbee had done, he noticed that the corks with which he had

stopped the ends of the tube to exclude the dust, seemed to

attract bits of paper and leaf-brass as well as the glass itself.

He surmised at once that this mysterious electricity, or

"virtue," as it was called, might be transmitted through other

substances as it seemed to 141n136b be through glass.

"Having by me an ivory ball of about one and three-tenths of an

inch in diameter," he writes, "with a hole through it, this I

fixed upon a fir-stick about four inches long, thrusting the

other end into the cork, and upon rubbing the tube found that the

ball attracted and repelled the feather with more vigor than the

cork had done, repeating its attractions and repulsions for many

times together. I then fixed the ball on longer sticks, first

upon one of eight inches, and afterwards upon one of twenty-four

inches long, and found the effect the same. Then I made use of

iron, and then brass wire, to fix the ball on, inserting the

other end of the wire in the cork, as before, and found that the

attraction was the same as when the fir-sticks were made use of,

and that when the feather was held over against any part of the

wire it was attracted by it; but though it was then nearer the

tube, yet its attraction was not so strong as that of the ball.

When the wire of two or three feet long was used, its vibrations,

caused by the rubbing of the tube, made it somewhat troublesome

to be managed. This put me to thinking whether, if the ball was

hung by a pack-thread and suspended by a loop on the tube, the

electricity would not be carried down the line to the ball; I

found it to succeed accordingly; for upon suspending the ball on

the tube by a pack-thread about three feet long, when the tube

had been excited by rubbing, the ivory ball attracted and

repelled the leaf-brass over which it was held as freely as it

had done when it was suspended on sticks or wire, as did also a

ball of cork, and another of lead that weighed one pound and a

quarter."

Gray next attempted to determine what other bodies would attract

the bits of paper, and for this purpose he tried coins, pieces of

metal, and even a tea-kettle, "both empty and filled with hot or

cold water"; but he found that the attractive power appeared to

be the same regardless of the substance used.

"I next proceeded," he continues, "to try at what greater

distances the electric virtues might be carried, and having by me

a hollow walking-cane, which I suppose was part of a fishing-rod,

two feet seven inches long, I cut the great end of it to fit into

the bore of the tube, into which it went about five inches; then

when the cane was put into the end of the tube, and this excited,

the cane drew the leaf-brass to the height of more than two

inches, as did also the ivory ball, when by a cork and stick it

had been fixed to the end of the cane.... With several pieces of

Spanish cane and fir-sticks I afterwards made a rod, which,

together with the tube, was somewhat more than eighteen feet

long, which was the greatest length I could conveniently use in

my chamber, and found the attraction very nearly, if not

altogether, as strong as when the ball was placed on the shorter

rods."

This experiment exhausted the capacity of his small room, but on

going to the country a little later he was able to continue his

experiments. "To a pole of eighteen feet there was tied a line of

thirty-four feet in length, so that the pole and line together

were fifty-two feet. With the pole and tube I stood in the

balcony, the assistant below in the court, where he held the

board with the leaf-brass on it. Then the tube being excited, as

usual, the electric virtue passed from the tube up the pole and

down the line to the ivory ball, which attracted the leaf-brass,

and as the ball passed over it in its vibrations the leaf-brass

would follow it till it was carried off the board."

Gray next attempted to send the electricity over a line suspended

horizontally. To do this he suspended the pack-thread by pieces

of string looped over nails driven into beams for that purpose.

But when thus suspended he found that the ivory ball no longer

excited the leaf-brass, and he guessed correctly that the

explanation of this lay in the fact that "when the electric

virtue came to the loop that was suspended on the beam it went up

the same to the beam," none of it reaching the ball. As we shall

see from what follows, however, Gray had not as yet determined

that certain substances will conduct electricity while others

will not. But by a lucky accident he made the discovery that

silk, for example, was a poor conductor, and could be turned to

account in insulating the conducting-cord.

A certain Mr. Wheler had become much interested in the old

pensioner and his work, and, as a guest at the Wheler house, Gray

had been repeating some of his former experiments with the

fishing-rod, line, and ivory ball. He had finally exhausted the

heights from which these experiments could be made by climbing to

the clock-tower and exciting bits of leaf-brass on the ground

below.

"As we had no greater heights here," he says, "Mr. Wheler was

desirous to try whether we could not carry the electric virtue

horizontally. I then told him of the attempt I had made with that

design, but without success, telling him the method and materials

made use of, as mentioned above. He then proposed a silk line to

support the line by which the electric virtue was to pass. I told

him it might do better upon account of its smallness; so that

there would be less virtue carried from the line of

communication.

"The first experiment was made in the matted gallery, July 2,

1729, about ten in the morning. About four feet from the end of

the gallery there was a cross line that was fixed by its ends to

each side of the gallery by two nails; the middle part of the

line was silk, the rest at each end pack-thread; then the line to

which the ivory ball was hung and by which the electric virtue

was to be conveyed to it from the tube, being eighty and one-half

feet in length, was laid on the cross silk line, so that the ball

hung about nine feet below it. Then the other end of the line was

by a loop suspended on the glass cane, and the leaf-brass held

under the ball on a piece of white paper; when, the tube being

rubbed, the ball attracted the leaf-brass, and kept it suspended

on it for some time."

This experiment succeeded so well that the string was lengthened

until it was some two hundred and ninety-three feet long; and

still the attractive force continued, apparently as strong as

ever. On lengthening the string still more, however, the extra

weight proved too much for the strength of the silk

suspending-thread. "Upon this," says Gray, "having brought with

me both brass and iron wire, instead of the silk we put up small

iron wire; but this was too weak to bear the weight of the line.

We then took brass wire of a somewhat larger size than that of

iron. This supported our line of communication; but though the

tube was well rubbed, yet there was not the least motion or

attraction given by the ball, neither with the great tube, which

we made use of when we found the small solid cane to be

ineffectual; by which we were now convinced that the success we

had before depended upon the lines that supported the line of

communication being silk, and not upon their being small, as

before trial I had imagined it might be; the same effect

happening here as it did when the line that is to convey the

electric virtue is supported by pack-thread."

Soon after this Gray and his host suspended a pack-thread six

hundred and sixty-six feet long on poles across a field, these

poles being slightly inclined so that the thread could be

suspended from the top by small silk cords, thus securing the

necessary insulation. This pack-thread line, suspended upon poles

along which Gray was able to transmit the electricity, is very

suggestive of the modern telegraph, but the idea of signalling or

making use of it for communicating in any way seems not to have

occurred to any one at that time. Even the successors of Gray who

constructed lines some thousands of feet long made no attempt to

use them for anything but experimental purposes--simply to test

the distances that the current could be sent. Nevertheless, Gray

should probably be credited with the discovery of two of the most

important properties of electricity--that it can be conducted and

insulated, although, as we have seen, Gilbert and Von Guericke

had an inkling of both these properties.