444 RECENT PROGRESS IN PHYSICS. 



(- + -) (1+- + -)^^ 



Biess found this formula also confirmed by liis experiments. 



To introduce an air thermometer into the branches without 

 changing the circuit in other respects, platinum wires were placed in 

 the branches with the same connecting pieces, and of equal length 

 and thickness with the platinum wire in the thermometer, so that 

 these pieces of wire could be removed from the branches and the 

 thermometer substituted for them ; the place in the circuit where the 

 thermometer stood was occupied by a connecting wire of equal dimen- 

 sions. 



In all these experiments the branches were very short, and it is 

 for such cases only that the above formulas are applicable. When 

 the branches are long, each induces in the other lateral currents in 

 the same direction. But if the main current in a induces a lateral 

 current in /9, a completes, as it were, the circuit for the lateral cur- 

 rent ^3 ; the lateral current excited in /? will thus traverse a in a direc- 

 tion opposite to that of the main current ; to this is to be added the 

 lateral current excited in a by /9. The effect of these lateral currents 

 is shown not only in the branches, but they modify the main current 

 in the general conductor. These exceedingly complicated disturb- 

 ances of the discharge current in a branched wire are difficult, as 

 Riess has justly remarked, to bring under a generally valid law. 



§ 48. Electrical retarding power of metals. — Riess concludes the 

 investigation just described by an account of his highly interesting 

 and important labors on the electrical retarding power of metals. 



We have seen that a wire brought into the circuit by means of 

 Henley's discharger retards the discharge, and that in consequence of 

 this retardation the depression of the air thermometer diminishes. 



The wire in the thermometer remaining unchanged, if we intro- 

 duce first a platinum wire, and afterwards one of copper of equal 

 length and thickness into the circuit, an equal depression will not be 

 obtained ; whence it follows that these wires, though they have the 

 same dimensions, do not retard the electrical discharge in a like 

 measure ; hence the retarding force of the two metals is specifically 

 different. 



With a copper wire a greater depression will be obtained than with 

 a platinum wire of equal length and thickness ; the copper, therefore, 

 retards the electrical charge less than the platinum wire. 



For discussion and computation of the retarding power of different 

 metals, the following is the simplest method to be pursued : First 

 place a platinum wire in the discharger and determine the depression 

 produced by a given charge of the battery. Introduce another wire 

 instead of the platinum, (having the same thickness,) and lengthen 

 or shortf^n it until tlie same charge of the battery produces the same 

 effect. The retarding forces are to each other inversely as the length 

 of the wires used. 



