350 



EECENT PROGRESS IN PHYSICS. 



B^P^ 



I have reported Riess' researches without interrupting the course 

 the narration by speaking of what has been done by others on the sa 

 subject. Let us now turn to these labors. 



§ 68. Knochenhauer' s researches on the current. — In a second article, 

 with the title ^^ Experiments on Latent Electricity^" {Versuche iiber 

 die gehundene,Elektricitdt, Pog. Ann., LVIII, 391,) Knochenhauer pre- 

 sents the law according to which the force of the secondary current 

 decreases when the distance from the main wire increases. 



Riess has shown, as already mentioned, § 61, that the force of the 

 secondary current decreases in the same proportion in which the axial i ' 

 distance of the secondary wire from that of the main wire increases. 



Knochenhauer thinks this law is " evidently insufficient." 



Starting, apparently, from the idea that the lateral current is a phe- 1 

 nomenon of induction, Knochenhauer attempts to apply here his law.* 



That a law stating the relation between action and distance, adapted 

 to the case of spherical bodies only, in which all action can be consid- 

 ered as starting from a single point, cannot hold good for wires run- 

 ning parallel to each other does not stop Herr Knochenhauer. His 

 law has such an astonishing elasticity that, by barely changing the co- 

 efficient, it serves for the secondary current. In his opinion there sub- 

 sists between the force of the secondary clirrent (measured by the air 

 thermometer) and the distance of the wire the relation 



6 = AaVnr~ 



in which 6 denotes the temperature of the thermometer in the second- ; 

 ary wire, and n the distance of the secondary from the main wire. 



This n, however, is not the axial distance, but the distance of the 

 wire in the clear, in which he assumes three lines as unity ; hence 

 the magnitude of 7i has first to be computed from the axial distance 

 a given by Riess. 



He first compares his formula with the results found by Riess. A 

 series of these observations he arranged in the following table, with 

 the values computed by his formula : 



In fact the values observed and those computed by the above form- 

 ula correspond sufficiently well by making A= 0.401, a= 0.489. 

 Indeed, the formula answers for very short distances, for which the 

 law of Riess, on evident grounds, is no longer applicable. 



But does this accordance of Knochenhauer's formula with the observa- 



*See Report of 1856. 



