Non-arcing metals.

rods with metallic vapor, which has a much lower resistance than the air. If the vapor is of high resistance, the amount of heat developed will be greater than if it is of low resistance and as a consequence more metal will be vaporized. If the resistance of the vapor is very low the amount of heat developed will be correspondingly low, and therefore the amount of metal vaporized will be comparatively small.

With a metal which gives a high resistance vapor, the heating is prevented from becoming very great from the very fact that the high resistance cuts down the strength of the current, and with the low resistant vapor the heat is prevented from becoming very small from the fact that he low resistance causes the current to increase in strength. Thus we see that while the high resistance vapor acts to confine a large amount of heat within a small space, it also acts to keep down the total amount of heat developed by reason of not permitting the current to rise to a considerable strength. On the other hand, the low resistance vapor enables a large current to flow, and thus a large amount of heat to be developed, but this is distributed over a much larger volume of vapor.

From these facts it follows that if the vapor is of high resistance, the arc will be of small crossection, which will help to increase its resistance, while if the vapor is of low resistance the arc will be of large crosssection which will help to reduce its resistance. The conclusion we are naturally led to, therefore, is that the higher the resistance of the vapor of a metal, the smaller the arc sustained between the ends of rod made of that metal. If two metals whose vapor has the same specific resistance are of different fusibility, then the two which melt at the highest temperature will be vaporized, the

Non-arcing metals.

least with a given amount of heat, therefore, the resistance of the arc produced by it will be the greatest. We therefore see that a non-arcing metal is one which does not vaporize readily, and whose vapor has a very high resistance. Experimentally it is found that zinc and antimony are about the best non-arcing metals we have. Now both of these fuse at low temperature, so that the only conclusion we can come to is that the resistance of their vapor is sufficiently high to overbalance the low fusibility.