of Small Inductances and Capacities. 591 
to within say 2 per cent. and adjusted to be of required value 
by varying the length and diameter of the rod. A series of 
tuning inductances can in this manner be easily made, which, 
when associated with known capacities, give circuits having 
known oscillation frequencies. 
When the coils have a smaller dimension-ratio, being only 
6 or 7 diameters long, then the above rule always gives, as 
it should do, an inductance value which is too large, but even 
in the case of such short coils not by very many per cent. 
This may be seen from the inductance measurements made 
by the telephone method on four short coils called for dis- 
tinction A, B, C, and D, which had the following dimensions 
and windings. The coils were made of silk-covered no. 36 
or no. 38 wire wound on glass tubes. 



TaBueE IT. 
_ Length of | Diameter of | Number of | pence eae ) 
) Coil. | Solenoid Solenoid Windings | eo =Lfrom | 
| — = 7! i the formula | 
| L=(xDé)(#DN). | 

| 
aes 205ems. | 33ems. 513 1,379,800 ems. | 
| 2 204ems. 35 ems. 813 | 3,918,200 ems. 
i J 20°8 ems. 34 ems 847 | 3,935,200 cms. | 
‘i ee 20°9 ems. 37 ems. 850 | 4,670,800 cms. 


The inductance of these coils was measured with the 
bridge and telephone, using a capacity of 0°00272 microfarad 
and the bridge values were as in Table III. (p. 592). 
The mean “values of the observed inductances of each coil 
differ from the extreme values in some cases by less than 
1 per cent., and in no case by more than 2 per cent. The 
mean observed values are less (as they should be) than the 
— calculated by the formula L=(rD#)(7DN) by about 
6 or 7 per cent. 
It le always been found that when the dimension ratio is 
as much as 50:1 or more, then there is a close agreement 
between the observed and calenliied value of the induchinds: 
The above described telephone and buzzer modification of the 
Anderson method can be therefore used to calculate the value 
of the capacity used in the bridge, assuming the calculated 
value of the inductance of the inductive arm. ‘Thus, if in 
the measurements recorded in Table I. we take the inductance 
