ii6 



NATURE 



[November 30, 1893 



into operation, deflection of the mercury stream occurred. It 

 was the object of the experiment to determine the performance 

 of the stream under the sudden fluctuations of the field which 

 occurred when the Holtz machine was under rapid discharge. 

 =5 Fig. 5 is from a photograph taken when nearly one hundred 

 sparks a second were passmg between the poles. 



Other photographs were obtained in a similar manner, the 

 deflecting forces, however, being due to the action between the 



Fig. 2. 



lines of a stationary magnetic field and those of an alternating 

 current traversing the mercury column. The arrangement of 

 the apparatus is shown in Fig. 6. The mercury stream was 

 introduced into the circuit of the alternating current dynamo, 

 already made use of in the experiments upon Froehlich's method. 

 It flowed through a strong magnetic field with horizontal lines. 

 The transverse oscillations of the mercury under these con- 



ditions were very apparent. When photographed by means of 

 a cxmera with optical axis parallel to the lines of force, the 

 stream strongly illuminated from behind and viewed through a 

 narrow horizontal slit, as in the previous experiment, a sinusoidal 

 trace was obtained. All the com plexities of the telephonic 

 trace disappeared ii these records, and curves corresponding to 

 thise of themjthol of instantane ous contact were always pro- 



.^ 



■o o- 



duced. The experiment was made by Mr. Henry Floy, to 

 whose efforts the photographs by Froehlich's method are also due. 

 This method has not been further developed. I introduce it here 

 to show that increased accuracy of record may be looked for as 

 the result of reducing in any practicable manner the mass of the 

 indicating device. 



NO. 1257, VOL. 49] 



Another attempt to record single periods in dynamo-electric 

 work should be mentioned here. It is described by Moler^ 

 in a recent paper. By means of a D'Arsonval galvanometer 

 with a period of vibration of a few thousandths of a second 

 curves of varying potential are traced, which show excellent 

 agreement wzth measurements by the method of instantaneous 



contacts. The instrument is not free from the errors due to 

 inertia. It is reliable only in recording changes of period con- 

 siderably greater than its own, but its use is a step in a direc- 

 tion along which progress may he looked for. 



Thus far I have dealt with methods of studying periodic 

 changes, the time-elements of which are easily within reach 



'<l 



Fig. 6. 



through experimental devices. I might have devoted myself 

 with as good reason to the consideration of recent advances in 

 the study of electrical oscillations of a higher order of frequency. 

 This is a department of physics in which much has been done of 



" A Dynamo-Indicator, or Instantaneous Curve-writing Vohmeter." 

 (Trans. Am. Elec. Engineers, vol. ix.: p. 223.) 



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