Construction of Non-inductive Resistances, 



189 



thicker wire, it follows that this method of constructing a 

 resistance not only greatly reduces the inductance, but also 

 enables a much larger current to be carried for a given 

 variation of resistance produced by the current. Hence for a 

 given current and for a given permissible rise of temperature 

 finer wire can be used ; the coils can therefore be made shorter, 

 and the inductance for this reason still further lessened. 



Lastly, even when such right- and left-handed spirals are 

 traversed by a direct current, much less disturbance is pro- 

 duced in a neighbouring delicate galvanometer than if a 

 singly-wound spiral resistance be employed ; so that these 

 right- and left-handed spirals of bare wire are valuable in the 

 construction of resistance-frames for large direct as well as for 

 large alternating currents. 



A number of portable resistance-coils have been constructed 

 in this way for general use in the laboratories of the Central 

 Institution. One of these is seen in fig. 3. It has also been 

 found convenient to fit up a number of stationary right- and 

 left-handed concentric spirals of platinoid in the dynamo 

 laboratory, for use in the regular experiments with alternate 

 currents. 



Table I. gives the particulars of some portable sets of stand 

 coils in the laboratories of the Central Institution, the second 

 three of which have been composed of right- and left-handed 

 spirals in the way just described and illustrated in fig. 3. 



Table I. 

 Particulars of Portable Stand Coils. 



Mark 

 on the 

 Stand. 



Kesist- 

 ance, in 

 Ohms. 



Number of 

 Spirals in 

 the Set. 



Length 

 of each 

 Spiral. 



Number of Con- 

 volutions in each 

 Spiral. 



Outside diameter 

 of the Convolutions. 



Size of 

 the Wire 



(S.W.G.). 



1 



2 



3 



4 



5 



6 



1-8 



2-54 



8-59 



5-0 

 99 



6 

 12 

 12 



20 double, 

 that is 40 

 coils in all. 



inches. 

 18| 



16^ 

 17 



78 

 63 

 84 



inches. 



Hi 



It 3 * 

 ItV 



No. 12 

 No. 10 

 No. 13 



No. 13 

 No. 15£ 



No. 27 



Outer. 



Inner. 



Outer. 



Inner. 

 inches. 



7. 



4 



15 

 27 

 54 



21 

 37 

 79 



inches. 

 \l 



n 



i 



