Tlie Use of Variable Mutual Inductances. 155 
We are dealing in fact with just that degree of isochronism 
between the free periods of the spherical sheet of air and the 
periods of the (thermal) disturbing agency for which our 
assumption of no dissipation ceases to represent even approxi- 
mately the actual conditions. 
It may be remarked, however, that the value of Bp given 
by (32) indicates a semi-diurnal barometric amplitude not 
widely different from the diurnal, provided equally favourable 
resonance conditions may be assumed. If the agreement of 
period between free vibrations and disturbing agency were 
somewhat better for the second term of Bp than for the first, 
the greater prominence of the semi-diurnal period might 
perhaps be accounted for. 
A less remote representation of atmospheric conditions 
might have been realized by introducing into the value 
of BH/(^ an additional factor, a function of co-latitude and 
longitude, so as to take some account of the fact that the 
diurnal variation of temperature is greater over a continent 
than over an ocean. Such a factor, however, is not competent 
to introduce into the barometric fluctuations at a given station 
terms whose periods are submultiples of that of the thermal 
excitation. 
XI. On the Use of Variable Mutual Inductances. By 
Albert Campbell', B.A* (From the National Physical 
Laboratory.) 
[Plate IX.] 
Contents. 
1. Introductory. 
2. Mutual versus Self Inductance. 
8 & 4. Construction and Adjustment of Variable Mutual Inductances. 
o, 6, 7 & 8. Uses of the same for measuring unknown Mutual 
Inductance, Self Inductance, and Capacity. 
9. Appendix. The Scale of the Excentric-Coil Mutual Inductance. 
1. Introductory. 
THE accurate measurement of small self inductances and 
capacities has of ]ate years assumed an increased 
importance owing to its direct practical application in con- 
nexion with wireless telegraphy ; and much important and. 
interesting work has been done on the subject especially as ; 
regards the calculation and measurement of seif inductance. 
For some time past I have found that suitably designed 
mutual inductances are of the greatest assistance in such 
measurements, and I incline to think that by their use some 
* Communicated by the Physical Society : read October 25, 1907. 
