190 



Construction of Non-Inductive Resistances. 



Table II. gives the results of experiments made on the sets 

 marked 2, 3, 4, 5, 6 by means of the secohmmeter ; the set 

 marked 6 being tested when each pair of coils were joined up 

 in parallel so as to increase the inductance as well as when 

 joined up so as to diminish it. 



Table II. 





Portable Platinoid Spiral Eesistances (fig. 



3), 



Mark 

 on the 

 Stand. 



Winding. 



Eesistance, 

 in Ohms. 



Inductance, 

 in Secohms. 



Time-constant, 

 in Seconds. 



2 ... 



3 ... 



Set of Singly-Wound Spirals ... 



Set of Single- Wound Spirals . . . 



Set of Doubly- Wound Spirals ; 

 Current circulating in all in 

 the Same Direction 



2-54 

 8-59 



1 98-1 



0-000059 

 0-00015 



00021 



23x10-6 

 18x10-6 



21xl0~ 6 





«{ 



*{ 

 ■{ 



Same set of Doubly-Wound 

 Spirals; Current circulating 

 in Opposite Directions 



1 99 



} „ 



I 5-0 



0-00013 

 0-000004 

 0-0000094 



l-3xl0~ 6 

 2-8X10 -6 

 1-9X10" 6 



Set of Doubly-Wound Spirals; 

 Current circulating in Oppo- 

 site Directions , 



Set of Doubly-Wound Spirals ; 

 Current circulating in Oppo- 

 site Directions 



The mean time-constant, therefore, of one of our sets of 

 spirals, whether singly-wound or doubly-wound, provided that 

 the current circulates in all the convolutions in the same 

 direction, is about 21 x lO"^ 6 ; whereas if the spirals be doubly- 

 wound and the current be circulating in opposite directions 

 in the two sets of spirals, the mean time-constant is about 

 2 x 10~ 6 , or about one tenth of the preceding. 



As there happens to be in the laboratory a solenoid, the 

 silk-covered copper wire on which is wound in a series of 

 concentric sections, it was thought interesting to compare the 

 time-constant of two of the sections joined up in parallel, 

 when the current circulated round both in the same direction, 

 with the time-constant when it circulated in opposite direc- 

 tions. Figs. 4 and 5, Plate IV., show the direction of the 

 current in the two cases. 





Eesistance, 

 in Ohms. 



Self-induction, 

 in Sechoms. 



Time-constant 

 in Seconds. ' 



Currents flowing round both coils 1 

 in the same direction (fig. 4) ... J 



Currents flowing round the coils \ 

 in the opposite direction (fig. 5) J 



0-346 

 0-346 



0-00059 

 0-000024 



17X10" 6 

 0-69 Xl0~ 6 



