266 Prof. Riess on Electric Pauses. 



much as it shows itself in other experiments which are frequently 

 made. The absence of the sparks in the interval of the pause 

 has been ascribed to the circumstance, that the portion of the 

 spherical surface nearest to the cone, possesses a greater density 

 than that necessary for the production of a spark. Now the 

 electricity developed on the conductor has not attained this den- 

 sity instantaneously, but gradually, and there must bave been a 

 point in which the sphere possessed the exact density necessary 

 for a spark. The question is, why did not the spark make its 

 appearance at this particular time ? The same question may be 

 asked with reference to experiments which are better known. A 

 large sphere on the conductor of a machine gives sparks to an 

 approximated conductor ; a small sphere gives a brush. When 

 through the slow turning of the machine a brush is produced at 

 a particular point of the conductor, this may frequently be trans- 

 formed by quicker turning into a glow. A smooth sphere which 

 furnishes sparks, gives brushes when it is roughened, and so forth. 

 In all these cases the same question suggests itself, why a phse- 

 nomenon of higher density appears, and that of lower density 

 disappears, while that of intermediate density does not occur, 

 although the density has been gradually increased. This diffi- 

 culty M. Riess believes renders the assumption necessary, that, 

 for the production of a spark, not only is a definite density i-equi- 

 site, but that this density must exist for a certain time. Accord- 

 ing to this, the spark first breaks out shortly after the electrodes 

 have attained the density necessary to its production. This time 

 may be small, but it is at all events very great in comparison 

 with the time required by the electricity to arrange itself upon a 

 good conductor; and thus it is not surprising that at a place 

 which has attained to a higher density, instead of the spark 

 another mode of discharge should make its appearance. In the 

 same manner, time is necessary for the brush and the glow to 

 be produced ; and it must be stated generally, that an obstacle 

 is to be overcome before the electricity upon the surface of a con- 

 ductor can propagate itself to the nearest layer of the limiting 

 medium, and that this overcoming requires time. This assump- 

 tion agrees well with the fact of the resistance to the passage of 

 an electric current from one medium into another. 



In the experiments on the pauses, the pause-sphere receives, at 

 all distances of the cone, more electricity than is necessary to 

 produce the density requii'cd by a spark ; but a portion of this 

 electricity is lost by streaming out. At smaller distances of the 

 electrodes the portion streaming out is of great density, but only 

 of limited extent ; and the density necessary for a spark is but 

 small ; hence the density of the sphere does not sink lower than 

 what is required for the appearance of the spark. At the great- 



