4 Prof. A. Battelli and Mr. L. Magri on 



Without discussing at length Boys's * experiments (their 

 main object being a didactical one), where the spark was 

 photographed by means of a rapidly turning objective, we 

 rather wish to point out those undertaken by Mieslerf, with 

 a view to test Thomson's formula. He photographed the 

 spark due to the discharge of some leyden-jars by means of a 

 lens and of a plane mirror set rotating by clockwork, using 

 a circuit formed by several brass spirals : the periods found 

 ranged from 0*000016 to 0*0000052 sec, which agrees well 

 with theoretical values. 



But specially remarkable on account of the favourable ex- 

 perimental conditions and the accuracy of the measurements 

 are the researches carried on by Lodge and Glazebrook I, 

 who, using an air-condenser and an induction-coil of great 

 self-induction but small resistance, photographed the spark 

 of discharge on a rotating plate, the velocity being capable 

 of being maintained constant and being measured with great 

 accuracy. Though Lodge and Glazebrook's measurements 

 are relative to very slow oscillations only (from ^ to j^sec.)? 

 yet their results are of special importance, as the single 

 determinations exceed in accuracy those of all previous 

 experimenters. 



In order only to show the rapidity of oscillations so far 

 reached in those spark photographs, we will mention that 

 much shorter periods (of the order . 00 q 00q sec.) have been 

 investigated by Decombe §, who photographed the spark of 

 a Hertz resonator by combining a lens and a rotating mirror; 

 and Trow^bridge and Duane ||, in a paper where the velocity 

 of transmission of electrical oscillations in metallic wires was 

 tested, photographed, by the usual method of the rotating 

 mirror, sparks of a period of about 2 X 10 -7 sec. 



5. Experiments performed by the method of charge and 

 discharge curves. — The values of the period of oscillation may 

 conveniently be derived from the form of the curves repre- 

 senting the behaviour of the charge or discharge of a con- 

 denser, both of them taking place through a circuit containing 

 a resistance and a self-induction. 



Among the most accurate researches made with this method 

 there are to be quoted Hiecke's'^f measurements, whose results 

 agree perfectly with theoretical deductions. 



* Phil. Mag. xxx. p. 248 (1890). 



t Wien. Ber. xcix. II a, p. 579 (1890). 



X Cambr. Phil. Trans, xviii. p. 186 (1899). 



§ Compt. Rend, cxxvi. p. 518 (1898). 



|| Phil Mag. xl. p. 211 (1895). 



f Wien. Ber. xcvi. II. a, p. 134 (1887). 



