SCIENCE AND PRACTICE. 255 



1, 2, 2, 5, 10, 10, 20, 50, 100, 100, 200, 500, 1000, 1000, 



2000 and 5000 ; 



making a sum total, when all the plugs are out and the 

 current passes from terminal to terminal through all the 

 intervening lengths, of 10,000 units. 



This simple arrangement enables the operator to make the 

 resistance of his apparatus infinitely small by inserting all 

 the contact plugs, or to open any resistance in whole numbers 

 between one unit and ten thousand of them at pleasure. 



The lengths of wire b d, &c., are of German- silver, insulated 

 with two coatings of silk, sometimes further guarded from 

 the air by a protecting varnish, and wound double upon 

 bobbins of dry wood or vulcanite. The purpose of this 

 method of double winding is to avoid the effect of induction 

 currents in the bobbins on making or breaking contact with 

 the battery ; induction takes place, of course, but the currents 

 circulating everywhere in opposite directions, the effect is 

 eliminated. The coils or bobbins are arranged in a mahogany 

 case and the terminals put upon a thick slab of vulcanite. 

 There is a difficulty which must not be overlooked in using 

 these resistance scales, which is not met with in using the 

 Rheostat. It is the spring which the current makes on 

 changing the contact pegs. When measuring resistances of 

 insulation where the capacity of a jar for charge is present, 

 this evil is principally felt, and it becomes necessary to put 

 the galvanometer out of circuit before making any change, 

 in testing both the insulation and resistance of conductors of 

 long cables, in order to avoid the strong charge and discharge 

 currents which would otherwise alter the magnetism of the 

 needle. 



27. Eisenlohr's Resistance Column. Another form of 

 resistance scale is that arranged by Professor Eisenlohr. 

 Seven brass rings are fixed upon a cylinder of dry wood, at 

 equal distances from end to end. In the space between each 

 two of these rings is coiled a length of well-insulated wire, 

 one end being soldered to the upper and the other end to 

 the lower ring. The length in the first space has a resist- 

 ance equal to one German mile of telegraph wire, that is to 



