Variable Mutual Inductances. 
159 
where R t and R 2 include the resistances of the secondary 
coils. In order that the second condition may hold, it is 
necessary to introduce into one of the secondary branches a 
coil a whose self inductance can be continuously varied ; by 
alternate adjustments of Ri/R 2 and the self inductance of this 
coil, a balance is easily obtained. The fact that B, i and R^ 
are partly of copper coils is apt to introduce some inaccuracy. 
The copper resistance, however, can usually be largely 
<\vamped without losing too much of the sensitivity. 
In connexion with this method I may mention that ic is 
perfectly applicable to the case of comparing the mutual 
inductances between one primary circuit and two separate 
secondaries. This case is shown in fig. 4. 
Fig. 4. 
By this means a thorough intercomparison (or adjustment) 
can be made between the various sections of a subdivided 
mutual inductance. By further sets of secondary coils for 
higher multiples (10, 100, and so on) any desired range may 
be obtained. By stranding (or otherwise subdividing) the 
primary coils as well as the secondary ones a multiple-range 
inductance is the result. In all cases the subdivision can 
be effected by other means than stranding the wires, which 
is merely used to avoid the separate adjustment of the 
sections. The main objection to the subdivision by stranding 
is that it increases the distributed capacity of the coils. 
With coils of many turns this might be a serious drawback, 
but with the relatively few turns required for 1 millihenry 
the effect can only be very small and I have not yet noticed it in 
practice. It should be kept in mind that, as in all standard 
inductances, eddy currents should be prevented by using only 
highly stranded wire for the winding of every section and by 
avoiding the use of any metal parts near the coils. 
By varying the relative dimensions and positions of the 
two fixed primary coils CC (fig. 2) and the movable 
