Proceedings of the Institution of Electrical Engineers Volume 13 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1884 Excerpt: ...to compile a table in which the safe working current would be almost directly proportional to the weight per unit length of copper used for a standard thickness, because the surface for radiation and convection increases in nearly direct proportion as the weight, the thickness of course being constant. Mr. T. H. Blakesley: May I speak to one point in Professor Forbes's paper. In investigating the laws connecting the current with the rise of temperature, or with a difference of temperature between the outside surface and the air, I would point out that Professor Forbes neglected the question of the increase in the internal temperature, the temperature gradient as points nearer and nearer the centre are considered. When any body is emitting heat there is a temperature gradient inside, and the inside of any conductor giving out heat must be necessarily much hotter than the surface. This is very strikingly exhibited in the case of lead fuses. Very often in making experiments with lead fuses you will find that the fuse bursts in a curious way, and this is due to the interior of the lead conductor being at a much higher temperature than the outside. Lead is one of those metals which expand in melting, so that the inside exerts a bursting force on the outside, and that is why a lead fuse often explodes as it does. I have followed out Professor Forbes's reasoning on these bare conductors, taking this point into consideration, and I have arrived at a formula which I have applied to Professor Forbes's own experiments as he has given them, and it answers very well. The formula is C =-162 d31--00165 dl t t = difference in temperature between the surface and the external air in degrees centigrade. d = diameter in millimetres. I find that expresses very closely the actua...