340 RECENT PROGRESS IN PHYSICS, 



great distances, the heating of the lateral wire will be too little to he 

 observed. 



Ri( ss stretched two copper wires 10 feet 6 inches long parallel to 

 each other, (Pog- Ann., L, 7.) One of them was connected by means 

 of copper wires 6 feet long with the circuit of the battery ; the ends 

 of the other were connected by similar wires -with the platinum wire 

 of the thermometer. By changing the distance between the axes of 

 the parallel wires the thermometer showed that the current generated 

 hy the straight 2'>cirt of the conducting circuit of the battery in the paral- 

 lel wire decreoses in the proportion in ivhich the distarice of the axis of 

 the ivires ivicreases, provided the distance of the wires at the start is 

 not too small ; for if the wires approach within a certain limit the 

 heat produced increases in a less proportion than the distances 

 decrease. 



To obtain somewhat elevated temperatures by the secondary cur- 

 rent, wires of great length must be used, and the management of 

 these is very troublesome when they have to be stretched straight. 

 Hence, when only the generation of an intense secondary current is 

 desired, it is greatly preferable to wind the wires in a flat spiral, as 

 already described, (144.) 



The current which is excited by the main spiral in the secondary, is 

 weaker the further the spirals are apart ; but it is easily seen that be- 

 tween the strength of the current and the distance between the spirals 

 there cannot be a simple proportion, for any one part of the circuit of 

 the main spiral excites a current, not only in the curved part lying 

 nearest to it and on the same side, but also in the more remote part of 

 the curve, on the opposite side; the latter is indeed weaker, but it acts 

 against the former and diminishes its effect. But the proportion of 

 the two opposite currents evidently changes when the distance of the 

 spirals is changed. If the starting point is from very small distances 

 of the two spirals the strength of the secondary current at first increases 

 more slowly, but at a greater distance far more rapidly than the in- 

 crease of the distance of the spirals. 



§ 62. Action of adjoining closed conductors on the generation of the 

 secondary cwrewi. —Riess extended on the floor of a room three copper 

 wires, 0.55 line thick and 10 ft. 6 in. long, parallel to each other, 

 (Pog Ann., L, 12,) these wires being denoted respectively by A, B, 

 and C. The axial distance between A and B was 4.45 lines, that of 

 B and C 2.35 lines. 



The wire A was inserted in the conducting circuit of a battery ; from 

 the ends of the wire C copper wires six feet long led to the thermom- 

 eter, and consequently the secondary wire C included the thermometer 

 in its circuit. When B was removed the unit of charge gave a tem- 

 perature indication of 0.135 ; B being restored to its place nearly the 

 same temperature was indicated ; but when the ends of B were joined 

 by a copper wire 14 feet long only 0.094 was the temperature indi- 

 cated. Hence it follows that — 



The current generated in a secondary wire hy the conducting wire 

 of a battery remains unchanged lohen a wire with free ends lies betiueen 

 the tiuo ivires ; but the current is diminished if the intermediate wire is 

 closed upon itself. 



