OF ELECTRO-D"i'NAMIC FORCES. 509 



the length of the string ; for the observed duration of the spark 

 is: — 



Seconds. 

 0-0816 + 0-0035 

 0-0408 - 0-0063 

 0-0204 - 0-0017 

 0-0102 -0-0007 



The first part of the duration of the spark is thus exactly in 

 proportion to the length of the string ; but the second part is so 

 small that it may be considered as arising from error of observa- 

 tion, which was unavoidable. 



It is thus evident that the result obtained by Prof. Wheat- 

 stone, according to which the duration of the spark on dis- 

 charge by simple metallic conductors is infinitely short in com- 

 parison with that ascertained in the present case, is in direct 

 accordance with this result. 



D. Application of the Dynamometer to the measurement of 

 Sonorous Vibrations. 



When a rapid alternation of positive and negative currents 

 ensues in a conducting wire, the continued motion of the elec- 

 ti-icity becomes converted into an oscillation. An oscillation of 

 this kind cannot however be observed by means of a galvano- 

 meter (for instance, a sine- or tangent-compass), because in this 

 case the effects of the successive opposite oscillations destroy 

 each other. 



But the case is different with the dynamometer, in tl^ two 

 coils of which the direction of the vibration always changes 

 simultaneously, and in which the deflection obsers'ed is in pro- 

 portion to the square of the intensity of the current ; for it is 

 self-evident that the simultaneous change of the direction in both 

 coils can exert no influence upon the action, because in the dy- 

 namometer a negative current transmitted through both coils 

 produces a deflection towards the same side as a positive current 

 transmitted through both coils. The occurrence of the deflection 

 of the dynamometer to one side or the other does not, as in the 

 galvanometer, depend upon the direction of the transmitted cur- 

 rent, but merely upon the mode of connexion of the extremities 

 of the wires of both coils. 



But an electric vibration may be readily produced in a con- 

 ducting wire by a magnetized steel bar vibrating so as to produce 



