This is what Henry Cavendish writes on page 490 here
My next trials were, to see whether this effect was owing to magnetism. Now, as it happened, the case in which the arm was inclosed, was placed nearly parallel to the magnetic east and west, and therefore, if there was any thing magnetic in the balls and weights, the balls would acquire polarity from the earth; and the weights also, after having remained some time, either in the positive or negative position, would acquire polarity in the same direction, and would attract the balls; but, when the weights were moved to the contrary position, that pole which before pointed to the north, would point to south, and would repel the ball it was approached to: but yet, as repelling one ball towards the south has the same effect on the arm as attracting the other towards the north, this would have no effect on the position of the arm. After some time, however, the poles of the weight would be reversed, and would begin to attract the balls, and would therefore produce the same kind of effect as was actually observed.
[…]
We may be sure, therefore, that the effect in question could not be produced by magnetism in the weights; for, if it was, turning them half round on their axes, would immediately have changed their magnetic attraction into repulsion, and have produced a motion in the arm.
Why does Cavendish say “...magnetic east and west”? Isn’t magnetism north and south?
And why would a lead weight “acquire polarity” from earth? I understand this to mean that, the sphere would acquire south and north poles and become attractive as a magnet.
I’m not clear what is going on here. Can you explain Cavendish’s concerns? Thanks.
