156 Mr. A. Campbell on the Use of 
of the sources of error inherent in other methods are avoided. 
The chief method (B) here described was designed for the 
measurement of the small self inductances (1 to 200 micro- 
henries) used in wave telegraphy, and it has proved most 
useful in such work. Before I proceed to describe the 
designs and the methods of use which I have found valuable, 
let us consider and compare mutual and self inductances in 
relation to their capabilities in practical use. 
2. Mutual versus Self Inductance. 
For a number of reasons mutual inductances are more 
easily dealt with than self inductances. Those of invariable 
value, when properly designed, have the following 
advantages : — 
(a) The absolute values can be calculated with much more 
certainty from the geometrical dimensions, since the formulas 
for mutual inductance are of high theoretical accuracy while 
those for self inductance are much less satisfactory *. 
(6) Unless the conductors are highly stranded, the current 
distribution varies with frequency and in general the self 
inductance will also vary. By keeping the two circuits at a 
relatively large distance from one another the mutual induc- 
tance is practically free from this effect. 
(c) The effects of distributed capacity are probably less in 
mutual than in self inductances. In all cases the distributed 
capacity of one of the two coils can be made very small by 
sufficiently decreasing the number of turns in it (or opening 
them apart) while increasing the number in the other coil to 
keep the M constant. 
When the mutual inductance is of the variable type, it can 
always be designed so that its value can be varied continuously 
from negative to jiositive through zero. This is a very great 
advantage, for with variable self inductance standards the 
incapability of reaching a zero value is a distinct drawback. 
3. Construction of Variable Mutual Inductances. 
For short range standards the construction used by Lord 
Rayleigh maybe employed f. In this the primary coil is 
inside the secondary and can be rotated round a common 
diameter as in the Ayrton-Perry variable self inductances, 
and the whole positive range is comprised within 90° of 
turning, which gives a scale not nearly open enough for 
* For example see Rosa, Bull. Bur. Stands, vol. ii. p. 161 (1906) ; 
and Strasser, Ann. der Phys. vol. xviii. p. 763 (1905). 
t Phil. Mag. vol. xxii. pp. 469-500 (1886); Collected Papers, vol. ii. 
p. 155. 
