( 259 ) 



F'or the first ])ui'pose I used it witli good results ^) when instead 

 of the silv^ered quartz thread a thin metal wire was stretched between 

 the poles of the electroniagnet. It appeared to be very easy to make 

 the period of the vibrations of the string agree with that of the alter- 

 nating currents used. In a few. seconds one has increased or dimin- 

 ished the tensioji of the string accurately to the desired amount and 

 for my purpose neither the sensitiveness nor the certainty of the 

 reading left anytiiing to be desired. 



If it should be necessary to increase the sensitiveness, a vacuum 

 could be applied, by which one would be enabled to obtain less 

 damped vibrations even of the lightest quartz thread. It must be 

 remarked that a vacuum is not alwa^^s necessary for obtaining little 

 damped vibrations, especially when alternating currents of very short 

 period, e. g. of O.OOi second and less are used. For the greater the 

 tension of the quartz thread, the smaller the damping ratio becomes. 



II. The methods in which the deflection of the galvanometer must 

 preferably be aperiodic are distinguished as II A, those with slow, 

 and II B, those with quick deflection. 



II. A. Of those with slow deflection we clioose two examples : the 

 measurement of currents with great external resistance, such as is 

 applied for examining insulation resistances and the measurement of 

 curi'ents with small external resistance such as tiiermo-currents. 



In both these measurements deflections of long duration, e. g. of 

 10 to 20 seconds can be used with good result. Here the normal 

 sensitiveness of the quartz thread in the galvanometer, as it is now 

 mounted with strong air-damping in the Leyden laboratory, no long- 

 er plays a part. Under these circumstances the mass has only a 

 small influence on the movement of the thread and the velocity of 

 the deflection is chiefly determined by the amount of the damping. 

 This latter only depends on tlie friction of the air when insulation 

 resistances are measured. 



If by applying a vacuum the movement of the quartz thread could 

 be brought near the limit of aperiodicity and at the same time the 

 deflection could be made slow by sufficiently relaxing the tension of 

 the thread, an instrument would be obtained by which insulation 

 resistances could be measured, many thousands of times greater than 

 is now possible with the most sensiti\'e galvanometers. 



In the measurement of thermo-currents some of the good points of 



^) Ueber Nervenreizung durch frequente Wechselströme. Pplüger's Archiv f. d. 

 ges. Physiol. 82, S. 101, 1900. See also "Onderzoekingen" Physiol, Laborat. Leyden. 

 !2ud series IV and V. 



18* 



