712 Prof. Stroud and Mr. Oate.s on the Application of 



or of a large size. We are, of course, aware of the evil 

 effects of convection-currents upon such long pointers ; in 

 fact, in one form of movable coil (which \\q shall refer to 

 as B), consisting of 211 tnrns of No. 38 copper wire — 

 resistance = 30 ohms — wound on an aluminium rectangular 

 frame 5'5 cms. X 2'f) cms. and 0*6 cm. broad, these convection- 

 currents proved a great nuisance. This effect, however, was 

 almost entirely eliminated by surrounding the pointer by a 

 fixed brass tube which had been flattened very considerably. 

 The mark on the pointer only just protruded through the open 

 end of this tube. In a later form of coil (referred to as C), 

 consisting of 1508 turns of No. 42 copper wire — resistance 

 = 485 ohms — wound on a copper frame 5*5 cms. X 2*5 cms. 

 and 2 cms. broad, we can observe very well without this 

 protecting tube. However, in a final form of our instrument 

 Ave hope to have the magnetizing coils entirely outside the 

 case supporting the movable parts. It would, moreover, be 

 desirable to have the torsion-head which supports the coil 

 readily accessible from the outside. 



We shall consider (i.) the comparison of capacities of the 

 order of from 10"^ to 1 microfarad ; (ii.) the comparison of 

 capacities of the order of a few M.M.F.-'s ; (iii.) the com- 

 parison of an inductance of the order of a henry with a 

 capacity ; and (iv.) the measurement of an inductance of the 

 order of a few microhenries. 



Comparison of Capacities and Calibration of Capacity-hox. — 

 The method we have used is the well-know^n one of De 

 Sauty, the alternating current at 100 volts, from the Leeds 



Tm, 1. 



ig 5uf,|>|^ 



Plaif 



mains, being supplied to the bridge as shown in fig. 1. It 

 will be seen that as there is a capacity in each limb of the 

 bridge, the only currents traversing the high resistances Ei 



