On Pimm's Method of Measuring Self-Inductance. 439 



(4) The value 5*4 x 10 23 , given indeed in the Comptes 

 Jlendus for the constant N of Avogadro, resulted from an 

 error (the omission of a factor -|) in the calculation of ex- 

 periments which gave exactly 7'1 x 10 23 . I must apologize 

 for this error in calculation, which was rectified some days 

 later, and refer for the exact figures to my publications 

 subsequent to September 1908. It is clear that the difference 

 between 5'4 and 7*1, which would indeed be inadmissible, 

 has no bearing on the precision of the method employed, a 

 precision which is I believe entirely a question of patience 

 and of time. 



XLV. On Piranis Method of Measuring Self-Inductance. 

 By J. P. KuENEN, Ph.D., Professor of Physics, Leiden 



University 



* 



IN the September number of this Magazine Processor 

 Leesf gives a simplified proof of the formula for 

 Pirani's method of comparing an inductance and a capacity. 

 The object of this note is to show that the result may be 

 obtained in a much simpler way, owing to the fact that the 

 conditions prevailing in the method are much simpler than 

 appears to be recognized. 



First consider the effect of breaking the main current 

 and assume that the current in the main branch is instan- 

 taneously reduced to zero. Using the notation in Professor 

 Lees's paper and reckoning the integral transient current 

 from the moment that the current is opened, we have .r = 0. 

 The condition is that the galvanometer branch must be free of 

 integral current, i. e. ^ = 0. Applying Kirchhoff's second 

 law adapted to impulsive currents to the mesh GPR (com- 

 pare the diagram which is taken from Professor Lees's paper) 

 and remembering that G contains no electromotive impulse, 

 whatever the inductance may be, it follows that the current 

 in PR is zero, i. e. y=0, hence likewise by Kirchhoff *s first 

 law the currents in the conductors Q and L, the only wire 

 therefore which conveys electricity being the resistance r 

 which completes the condenser circuit. Finallv, we have 

 from either of the meshes SGQ or SRPQ 



L^/i = < Vi? , Kxr or L = Kr 2 . q.e.d. 



* Communicated by the Author. 



t C II. Lees, Phil." Mag. [6] xviii. p. 432 (1909). 



