400 THE POPULAR SCIENCE MONTHLY. 



ENCE Monthly, translated from the Comptes Hefidus, indorses this 

 view, and, as no editorial protest has been added, it may have a ten- 

 dency 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 supi:>oses that all electricity 

 produced by the ordinary frictional machine is static which most as- 

 suredly 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 cura- 

 tive 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 vol- 

 taic battery. 



Knowing that electricity at rest always tends to difiiise itself on 

 the surface, in fact, that it always confines itself to the surface, it be- 

 came, 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 ofi"ered 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 difierent 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 parti- 

 cles ; indeed, we know that it does so." 



In attempting to determine this question, Pouillet and others seem 

 always to have used electricity prodixced by a voltaic battery; and 



* We give to the terms static and dynamic the old meanings, as evidently does the 

 writer under review. According to the new definitions suggested and advocated by 

 Profs. Tliomson and Tait, dynamics includes statics. The point is one which does nol 

 affect the main question, however. 



