492 Prof. J. A. Fleming. On an Instrument for the [Jan. 26, 



a metre or a metre and a-half in length and 10 cms. outside diameter, 

 the thickness of the walls being about 5 mm. On this tube is wound 

 as before an inductance coil of 100 or 200 turns of No. 14 S.W.G. 

 bare copper wire. Also the tube is provided with an outer jacket or 

 brass outer tube J (see fig. 3). This is conveniently formed out of 

 the sheet of thin brass bolted round the tube, and one end of the 

 inductance coil SS is attached to this jacket, the other end of the coil 

 being attached to a brass ring d, carrying an ebonite handle H. In the 

 interior of this ebonite tube, at one end, there is an inner brass tube I 

 which can slide easily in and out of the ebonite tube e. This brass 



FIG. 3. 







tube has a pin p fixed at one end, by which it is attached to a copper 

 bar LI, L 2 , L 3 , L 4 , ending in a curved crutch or collar K on which the 

 inductance spiral rests (see fig. 4). The other arrangements are as in 

 the instrument already described. 



The only differences between the forms shown in fig. 1 and fig. 4 

 are that in fig. 1 the outer metal jacket is movable and the inner one 

 is fixed, whereas in the second form, the outer jacket is fixed with 

 respect to the inductance coil, and the two together are drawn away 

 from the inner brass tube, thus reducing the capacity and inductance 

 at the same time. The instrument is very convenient for the measure- 

 ment of small inductances. Thus, for instance, suppose it is desired 

 to measure the inductance of a wire for high frequency currents, the 

 wire having an inductance of not more than a few hundred centi- 

 metres, that is to say, something of the order of a microhenry, this 

 inductance may be measured by the instrument in the following 

 manner: An insulated wire AB (see fig. 5) is laid alongside the 

 copper bar L 2 , L 4 , and the circuit of the wire is completed by a con- 

 denser C of known capacity and a spark gap S. Oscillations are then 





