Prof. A. Schuster's Electrical Notes, 175 



discharging of the condenser. The effect would be likely to 

 be much more marked if the capacity were much greater. 



The conditions for a maximum throw of the galvanometer 

 for a given E.M.F. are, moreover, not the same as those for 

 the maximum sensitiveness to steady current. There is 

 usually no difficulty, however, in obtaining sufficient sensi- 

 tiveness of galvanometer to detect want of proportionality of 

 the arms P Q R S of the bridge, so that a change of S by 1 

 in 200 will thus give a deflexion of convenient magnitude. 

 It is, however, very advantageous to have a galvanometer 

 whose period of oscillation can be varied at pleasure, so as to 

 give about the same throw as steady deflexion for the chosen 

 value of d$. One advantage which the method has is that 

 there is no necessity to know the galvanometer or battery 

 resistance. In the ordinary ballistic-galvanometer method it 

 is very doubtful if the resistance of the galvanometer, unless 

 it is measured at the time, is known within 1 to 2 per cento 



Further, the two essential parts of the determination 6 and 

 a may be made in quick succession, without any shifting of 

 contacts or connexions and with the battery-current flowing 

 continuously, leaving only the period of the galvanometer 

 and its damping correction to be afterwards determined. 



The method may be of service in the simultaneous deter- 

 mination of the resistance of, and joint capacity and inductance 

 of, a submarine cable or telephone- or telegraph-line. 



XVII. Electrical Notes. By Arthur Schuster, F.R.S* 

 II. On the Measurement of Resistance. 



A NUMBER of physical instruments, like the bolometer 

 and platinum thermometer, depend on an accurate 

 measurement of electric resistance. The question not un- 

 frequently therefore arises, what the limit of accuracy is with 

 which, with a given galvanometer and resistance, the measure- 

 ment can he made. • 



Whatever arrangement we adopt, the sensitiveness is always 

 increased by an increase of electromotive force ; and the 

 limit is reached either when we have put into action all the 

 electromotive force at our disposal, or when the currents 

 become so strong that there is danger of overheating one or 

 other of the resistances. The overheating may either damage 

 the insulation or produce disturbances in the way of thermo- 

 electric currents, or be the cause of other irregularities. For 

 a given resistance, we may generally fix on some current 



* Communicated by the Author. 



