(257) 



practicability of a galvanometer to know its normal sensitiveness. 

 This is calculated by tlie formula ') 



E, — '- (45) 



where E^ denotes the normal sensitiveness, 



a the deflection in millimetres, 



t the period of a whole oscillation ('~) in seconds, cal- 

 culated for undamped vibrations, ") 

 / the current in amperes, 

 V the microscopic magnification, and 

 w the internal conductive resistance in ohms. 



On account of an interesting paper by Walter P. White ■'') we 

 remark that the quantity normal i-iensitiveness is sharply defined in 

 a formula and hence need not give rise to misunderstanding. The 

 quantity mentioned may be very usefid for forming an idea of 

 changes wiiich may eventually be made in an existing galvanometer 

 or may advantageously be applied in the construction of a new 

 instrument. In the normal sensitiveness one has a valuable, important 

 datum about a galvanometer, but it is obvious that tlie practicability 

 of the instrument is still far from being determined by it. 



For when judging of the practicability a number of other properties 

 play an important part, as e.g. the internal resistance that can be 

 reached in practice, the amount of the damping, the constancy of 

 the zero point, the proportionality of the deflections to the currents, etc. 



The normal sensitiveness is 



for thread 10 10', 



„13 1,4X10', 



„14 2 X:IO«, 



„ ,, 20 (passingly mentioned in chapter 7) 2,1 X 10'. 



If we could succeed in making an aluminium wire of 1 ^x diameter 

 and 12.7 cm. length, i.e. of the same length as each of the three 

 first quartz threads, we should obtain a galvanometer of which the 



1) See formula (5) in Ann. d. Physik. 12. p. 10G3. 1903. 



2) In Ann. d. Phys. I.e. we denoted by t the duration of a complete oscillation. 

 This qualification implies that our formulae only hold for periodic motion. We 

 think it desirable to mention expressly here, that the period t must be calculated 

 for undamped oscillations. 



3) Walter P. White. Sensitive moving coil galvanometers. The Physical Review 

 vol. 19, n". 5, p. 305. 1904. 



18 



Proceedings Royal Acad. Amsterdam. Vul. VIII. 



