100 Mr. R. T. Glazebrook on a Method of 



the elasticity of the rod PJST makes it press against the 

 screw T, which is electrically connected with the binding- 

 screw R. When the current passes through the electromagnet, 

 the magnet attracts the iron attached to the rod PN and 

 brings it into connexion with the stop G, which is electrically 

 connected with the binding-screw S. The letters PES indi- 

 cate the same points in this figure as in fig. 1. All the places 

 where contact is made are covered with platinum, and the 

 whole arrangement is fastened down to an ebonite board. As 

 the current passes intermittently through the coils L M of the 

 electromagnet, the vibrating-piece PN strikes alternately 

 against the parts G and T. When it strikes against G, the 

 opposite plates of the condenser are connected with the two 

 poles of the battery; when it strikes against T, the condenser 

 is discharged." 



The main advantages of this method over the one ordinarily 

 employed for the determination of the capacity of a condenser 

 whose capacity is comparable with a microfarad are easily 

 seen. In the first place it is a null method ; no assumptions 

 are required as to the constancy of the battery. In the 

 second, the galvanometer can readily be made much more 

 sensitive. For accurate work in the ordinary method, the 

 field of force in which the galvanometer-needle hangs should 

 be nearly uniform. The earth's force is much too strong, 

 unless an enormous battery-power be employed, and it is 

 difficult to secure a uniform field by a control magnet. 

 Something may of course be done by the use of an astatic 

 galvanometer-needle; but it is impossible to increase the 

 sensitiveness in this way very much, because the torsion of the 

 suspending fibre then becomes important. Again, in the 

 ordinary method the time of swing cannot be determined 

 accurately if it be made too small; the moment of inertia of 

 the suspended parts therefore must be considerable, and the 

 deflection produced by a given quantity of electricity suddenly 

 discharged through the galvanometer is proportionately de- 

 creased ; for, if T be the time of swing, the throw produced 

 by a quantity of electricity Q, then T sin ^ 6 is proportional 

 to Q. In addition to this, there is the correction for damping 

 which has to be considered. 



On the other hand, objections may be raised to the method 

 which has just been described. The most important of these 

 is founded on the fact that the time during which the plates 

 of the condenser are in contact either with each other or with 

 the poles of the battery is very short. It may happen, then, 

 that the condenser is neither charged nor discharged com- 

 pletely at each vibration of the commutator; while if there be 



