112 Mr Laby, A string electrometer. 



till the portion AB (fig. 5) of the deflection potential curve is 

 realised. 



Characteristics of the String Electroscope. 



The fibre will stand vibration and sudden large changes of 

 voltage without breaking. The instrument has been carried on a 

 bicycle without damage to the fibre. 



Sensitiveness. No common standard of comparison has yet been 

 proposed for electroscopes. The unit of deflection used in this 

 paper is one division of the micrometer eye-piece : 1/5 of this 

 could be read with certainty. The closeness with which the 

 observations fall on curves shews that the unit chosen is not 

 unduly small. In fig. 4 (plates 6'6 mm. apart) a sensitiveness 

 of 50 divisions per volt is shewn. At the same time the curve 

 is nearly a straight line, i.e. the deflection is approximately pro- 

 portional to the voltage, and the p.d. between the plates was less 

 than 40 volts. The electrometer has been quite sensitive using 

 only 20 volts p.d. ; such a small p.d. could be kept constant more 

 readily (by the use of Weston cells, say) than the larger ones 

 usually required for electroscopes and electrometers. On the 

 other hand, large voltages, which require to be constant, would 

 generally still be required, for example, to saturate an ionised gas. 

 With the plates at 3'3 mm. apart and a p.d. of 36 volts a sensi- 

 tiveness of 100 divisions per volt over a range of "4 volt was 

 readily obtained. 



Oscillograph Uses. 



I do not think, however, that the value of the string electro- 

 meter will lie especially in its sensitiveness. It is not difficult to 

 make a gold leaf electroscope highly sensitive over a small range. 

 But there is a distinct need now for an electrometer capable of 

 automatically recording rapid changes of voltage, in the study 

 of discontinuous phenomena in the direction indicated by von 

 Schweidler*. For example in the beautiful experiment of 

 Rutherford and Geiger of counting the number of a particles, 

 which arrive in an ionisation vessel, by the excursions of an elec- 

 trometer the smaller the free period of the electrometer the 

 better. 



To test the oscillograph powers of the instrument, the plates 

 were placed 3 mm. apart at a P.D. of 120 volts, and the tension 



* Congres international poxir I'etude de la radiologie et de I'ionisation, Liege, 

 1905. 



