Dust Figures of Electrostatic Lines of Force

Extract

When lecturing on Electrostatics at the Glasgow and West of Scotland Technical College, the author endeavoured to impress upon his students the identity of the laws of force for magnetism and electricity at rest. He thought it would further this idea if he could get pictures of electrostatic lines analogous to those obtained from magnets by means of iron filings, and an analysis of the difference of the conditions in the two cases led to the conclusion that there was no reason why this should not be done.

When we shower iron filings upon a glass plate, or a sheet of paper, under which a magnet is placed, unlike poles are induced at the ends of each filing, and it becomes for the time being a little compass needle. Since it is very short, the total action upon it will be a couple turning it into the direction of the force at its centre, unless it be close to the magnet, in which case there will be, in addition, a translation along a line of force. This latter effect causes the filings to crowd towards the edges of the magnet.

Now, in electrostatics we also have an inductive action, opposite charges being induced in the near and far ends of each particle as it comes into the field, but we have the important difference, that whereas the magnetic poles are attached to the filing and cannot be separated from it, the electric charges will leave a body if they have an opportunity of doing so.