Page:Popular Science Monthly Volume 7.djvu/416

400, translated from the Comptes Rendus, indorses this view, and, as no editorial protest has been added, it may have a tendency to mislead many. Let us, therefore, consider the facts in the case.

In looking up the history of this subject, the first mistake that we meet is the confounding of static with dynamic electricity, or rather an utter ignorance of what static electricity is. The author of the note to which we have referred evidently supposes that all electricity produced by the ordinary frictional machine is static—which most assuredly is not the case. In making this mistake, however, he is not by any means alone. Dozens of writers have committed the same error, and it is not long since a medical man wrote a book on the curative powers of static, as distinguished from dynamic, electricity, while any physicist would have told him that in the entire volume there was not a single case described in which static electricity was used! Whenever electricity is in motion, that is to say, when it is flowing along a conductor, it is dynamic, no matter from what source it may be obtained. When at rest—that is, when it is in equilibrium—it is static. Dynamic electricity may be produced by the ordinary plate or cylinder machine; static electricity may have its origin in a voltaic battery.

Knowing that electricity at rest always tends to diffuse itself on the surface, in fact, that it always confines itself to the surface, it became, at an early period, a question whether electricity in motion did not follow the same law. Pouillet determined the question in a very ingenious manner. He took a cylindrical wire of a certain size and measured the resistance which it offered to a current of electricity. He then rolled the wire out flat and measured the resistance again; it was found to be the same, although it is evident that the extent of the surface of the conductor was by this means greatly increased. Other experimenters have determined the question by different methods, but always with the same result. The committee of the French Academy, which included Becquerel, De la Rive, Pouillet, and others, adopted a solid square bar as the best form for lightning-rods; and Sir William Snow Harris, though often quoted as favoring rods which present a large surface, says: "Provided the quantity of metal be present, the form under which we place it is evidently of no consequence to its conducting powers, since it would be absurd to suppose that a mass of metal, under any form, did not conduct electricity in all its particles; indeed, we know that it does so."

In attempting to determine this question, Pouillet and others seem always to have used electricity produced by a voltaic battery; and