How insulators and conductors were discovered? I know Stephen gray has discovered insulator and conductor. But I want to know that how did he know which one is conductor and which one is insulator ? Please tell me the details how had he done that above thing (that is conductor and insulator)
 A: He used for this purpose a glass tube
a few inches in diameter and length. A Cork had
been fitted into each end of the tube to keep out the dust.
He tried first to determine whether or not there was any
appreciable difference in the electrification of the tube with
and without the corks and found none. He did find, however, that when the tube was electrified the Cork would
attract and repel a feather.
In his next experiment he attached an ivory ball to the
end of a stick about four inches long and inserted the
other end of the stick in the Cork. When the tube was
rubbed, he found that the ball was electrified as the cork
had been. Carrying the experiment further, he attached the
ball to longer sticks and to brass and iron rods, with similar
results.
As the sticks and metal rods became longer, he experienced difficulty due to bending,
 and he conceived the idea of using a piece of pack thread or string attached to the cork
and to the ivory ball. With the longest pack-thread he could
manage by suspending the ball over the edge of a balcony
he was still able to transmit electrical charges to the ball. He
then tried suspending a longer pack-thread over a nail in a
beam to the ivory ball, but this time the experiment failed.
He surmised correctly that the charge had been led off
through the nail into the beam.Like this he attached so many materials in between glass tubes cork and ivory ball and those material which where conductors allow ivory ball to attract light object and those where insulator they don't allow ivory ball to attract light object.
A: He inserted cork on both the ends of the glass tube in order to keep dust away and then when he rubbed the glass tube with silk, he found glass tube attracting light objects(same as William Gilbert found in his experiment).
But,astonishingly,cork started attracting light object even if it is not rubbed from this he come to know that electricity is not static but it flows then he inserted a stick in the middle of the cork and the same attraction of light objects could be seen in that stick i.e the stick was attracting the light object.Now, he removed stick and inserted one end of the metal wire in the middle of the cork and at the    other end of the metal wire, an ivory ball and he found the same attraction by the ivory ball.But now when he inserted thread in place of metal wire ,he found that there was no attraction of light object  by the ivory ball.
Like this he attached so many materials in between glass tubes cork and ivory ball and those material which where conductors allow ivory ball to attract light object and those where insulator they don't allow ivory ball to attract light object
