18 Prof. E. EdluncPs Researches on the Passage 



have found to be the ease when the spark is formed in a space 

 filled with air*; while the opposite was presented at pressures 

 below 2*7 millim. : the deflections then became greater; that 

 is to say, the stronger current was that which traversed the 

 spark in the same direction as the disjunction-current. This 

 remarkable property of the spark in the rarefied-air space 

 would perhaps deserve a more special study by means of a 

 particular investigation. 



When the discharge-current passes through the galvano- 

 meter-coil, of course two induction-currents in opposite direc- 

 tions are produced in that coil also. For preventing the action 

 of these currents upon the magnetized needle it suffices to 

 insert a wire of sufficient resistance, as a bridge, before the 

 galvanometer-coil ; in fact, as the resistance of this bridge can 

 be made much less than the total resistance of the rarefied-air 

 space, the induced currents, which are of equal intensity but 

 opposite in direction, pass almost exclusively through the 

 bridge, and consequently cut off their action upon the mag- 

 netized needle. I convinced myself, by the following simple 

 procedure, that their effect upon the needle became really 

 insensible: — An induction-coil, of approximately the same 

 quality and size as that of the galvanometer, was placed at w. 

 The deflections were then seen to be considerably diminished 

 if the pressure was greater, but to be increased when the pres- 

 sure was less. When, on the contrary, a wire of suitable 

 resistance was placed as a bridge before the induction-coil, 

 the deflections became perfectly equal, whether the induction- 

 coil was or was not inserted in the circuit. As in all the 

 experiments the galvanometer-coil was always provided with 

 a bridge offering a suitable resistance, the induced current 

 exercised no sensible influence upon the results obtained. 



The electric current due to the passage of the discharge 



through the rarefied-air space has been expressed by — — , in 



conformity with the reasons given in the preceding pages. 

 In regard to this, it should be remarked that m (that is to say, 

 the resistance of the part of the circuit composed of wires) is 

 infinitely small in comparison with the resistance r-J, of the 

 rarefied air. The following experiment has proved that the 

 current in question cannot be calculated from the usual for- 



e 

 mula of Ohm -~ or since m can be neglected in compa- 

 rison with rd, by the formula — =• 



; J i\l 



* Ofoersigt af Kongl. Vetensk.-Akad. Forhandl. 1868; Pogg. Ann. 

 cxxxvi. j Ann. de Chim. et de Phys. [4] xvii. ; Phil. Mag. [4] xxxviii. 



