108 M. E. Wartmann on the Explosive Distance of the 



1 . Two straight tapering brass cones, the sides being 0*01 

 metre in length, and the base 0*008 metre in diameter. 



2. Two brass cones truncated at the summit, the base measu- 

 ring 19*18 millims. in diameter, and the truncation 2*82 millims.; 

 the apotheme, which is 16*35 millims. in length, is terminated 

 by a cylindrical embase 5*08 millims. in height. 



8. Two brass spheres 14 millims. in diameter. 



4. Two gold spheres 18*5 millims. in diameter. 



5. Lastly, two brass spheres 15 millims. in diameter, fixed to 

 the arms of a second universal discharger, which was sometimes 

 introduced into the circuit. 



When the RuhmkorfFs coil is at work, and one of the 

 electrodes is slowly separated from the other, a distance is 

 reached at which the spark ceases to pass. If the current be 

 then reversed by means of the commutator, the spark either passes 

 again, or the interval requires to be diminished before this occurs. 

 Sometimes the difference is not well marked at first, and requires 

 a certain duration of the electric current before it becomes ma- 

 nifest. When once produced, it remains tolerably constant as 

 long as the current of the battery retains its intensity. 



The apparatus may also be regulated by using the second dis- 

 charger. The electrodes which terminate the first are removed to 

 somewhat too small a distance, and the difference compensated 

 by opening the arms of the other excitor. When the interval 

 between the electrodes is too great for the passage of the spark, 

 it passes or ceases between the spheres of the secondary excitor, 

 according to the direction of the current. 



Of the terminal conductors, the small pointed ones appear 

 least favourable for experiment. The change occasioned by the 

 direction of the induced current may be limited to a variation in 

 the abundance of sparks. I have nevertheless succeeded in de- 

 termining their cessation at distances comprised between 0*033 

 metre and 0*052 metre. 



The proof is easier with the truncate cones and the spheres. 

 When the electrodes consist of well-polished brass, the passage 

 of sparks with high tension soon tarnishes them, from the for- 

 mation of black spots of oxide. The difference in their con- 

 ductibility which might result from this is not the cause of 

 the phenomenon ; for it remains after the electrodes have been 

 cleaned without altering their distance. Moreover on using the 

 gold spheres, the surface of which is not sensibly altered by the 

 discharges, the influence of the direction of the current upon 

 the striking-distance is shown in a very clear manner. 



Neither does this influence depend upon a difference in the 

 form of the electrodes which terminate the induced wire. This 

 may be shown by placing in the circuit a commutator which 



