30 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



pure substance has a very high resistance and the admixture of a 

 Very small quantity of another substance reduces its resistance 

 very greatly indeed. Thus a beaker of freshly distilled water (free 

 from air) in which are placed two clean platinum electrodes has a 

 resistance of, say, 25,000 ohms and the addition of one one-thou- 

 sandth of one per cent, of sulphuric acid reduces the resistance to 

 a few hundreds of ohms. 



16. The rheostat. An arrangement for inserting more or less 

 resistance into an electrical circuit at will is called a rheostat. 

 Figure 1 3 shows the usual arrangement of a rheostat. A number 

 of resistances rrrr are connected to terminal blocks of metal bbbbb 

 and a contact finger / of metal, broad enough to bridge over the 

 space between the adjacent blocks bb t is arranged so that it can 



be moved sidewise, thus connecting any number of resistances r 

 in circuit between the terminals A and B of the rheostat. The 

 resistances rrrr, Fig. 1 3, are usually made of metal of high 

 specific resistance so that the wire may be of moderate length 

 and yet large enough to be mechanically strong and to have suf- 

 ficient area to radiate the heat which is generated in it by the 

 current. One of the most satisfactory of these high resistance 

 metals is a nickel-steel alloy which is known in commerce under 

 the name of " Climax " metal or " Superior " metal. 



The so-called water rheostat which is frequently used consists 

 of two electrodes dipping into a vessel, or tank, containing a weak 

 solution of common salt. The current enters at one electrode, 

 flows through the salt solution, and leaves it at the other electrode, 

 and the resistance can be adjusted by varying the amount of salt 

 in solution or by moving the electrodes. 



