144 



ELECTRICAL MEASUREMENTS 



arrangement, but with the coils in straight lines. This greatly 

 facilitates keeping the top of the box clean. 



Multiple Decade Arrangements. Instead of arranging the 

 coils so that they are in series, they may be put in multiple if 

 given the proper values. Fig. 68C shows such an arrangement. 



FIG. 69. Feussner's and Smith's decade arrangements of resistance coils. 



Let the highest resistance to be obtained in the decade be 

 represented by A, or l 9>{oA, the next lower step is %o-A> this is 

 to be obtained by placing another coil in parallel with the first; 



by figuring the parallel circuits it 



^J_._ll i s seen that the second coil must 



I LJ<LOJ [ i / have a resistance of 9A. Simi- 

 larly 

 have 

 4.2A, 

 0.2A 

 used. 



the remaining coils must 

 resistances of 7.2A, 5.6A, 

 3.0A, 2.0A, 1.2A, 0.6A, 

 and 0. Ten plugs must be 

 This arrangement is most 

 advantageous when the total re- 

 sistance is low, for the compo- 

 nent coils have higher resistances 

 and are therefore more readily 

 adjusted than when the series ar- 

 rangement is employed. 



Arrangements for Reducing 

 the Number of Coils in a Dec- 

 ade. The economical disadvan- 

 tage of the original decade ar- 

 rangement lies in the large num- 

 ber of coils which must be made 

 and adjusted; several alternative 

 arrangements are shown above. 



In Feussner's decade arrangement, which gives resistances 

 from to 9 units, the first four coils are arranged as in the ordi- 

 nary decade system; the fifth value is obtained by using a single 



70 



