520 Messrs. W. De La Rue and H. W. Miiller on [Dec. 13, 



have commenced working with it systematically, and have gradually aug- 

 mented the number of cells ; we now possess 8040 in actual work, and 

 have 2680 more completed, but not charged with fluid. Amongst the 

 8040 cells now in use are the first 1080 constructed in 1874, experiments 

 with which we described on the 24th February, 1875*. Subsequently 

 from time to time we have communicated to the Society some of the 

 results we have arrived at, and in the detailed communication of which 

 the present is a short abstract we have given the full particulars of our 

 experiments. The ' paper in question deals mainly with the striking- 

 distance between terminals of different forms in air and in other gases at 

 ordinary atmospheric pressures, and in air at reduced pressures short of 

 the partial vacua of the so-called vacuum tubes. Besides these experi- 

 ments the paper describes the effects of currents of high tension in 

 inducing secondary currents, and also their effects in inducing mag- 

 netism. 



We have found that the discharge of the battery, with one or two 

 poles in the form of a point, presents several interesting phenomena 

 which precede the true jump of the spark, and which do not occur 

 with other forms of terminals — for example, disks or spherical surfaces. 

 With 8040 cells the striking-distance between a paraboloidal point, posi- 

 tive, and a disk is about 0-34 in. (8*64 millims.) ; but there is always a 

 luminous discharge, very apparent, far beyond the distance measurable 

 by our micrometer-discharger, namely 1*16 inch (29*5 millims.), as we 

 have before stated f. 



The current which passes during the luminous discharge which pre- 

 cedes the jump of the true spark is extremely feeble in comparison with 

 that which takes place after the spark has passed and the voltaic arc has 

 formed ; even when the point and disk are not more distant than :02 

 inch beyond the striking-distance (0*34 inch) for 8040 elements, it is only 

 3"5"Vt P ar ^ °f > moreover the current is diminished to °^ that °^ 



the arc when the point and disk are 1*16 inch distant. 



The appearance of the discharge is very different, according as the 

 point is positive or negative ; it is intermittent in both cases, but is much 

 less discontinuous when the point is negative than when it is positive, as 

 can be seen with a microscope having a rotating mirror placed in the bend 

 of the body, between the objective and eyepiece. The appearances 

 observed are shown in the wood engravings which illustrate the paper. 



"When a point and a disk 1*5 inch diameter are used as terminals, and a 

 band of glazed writing-paper 1*5 inch wide, and say 0-00425 inch thick, 

 is placed on the disk, a very strong adhesion of the paper to the disk 

 taker place, and it requires a very strong pull, when 8040 cells are em- 

 ployed, to make the paper slide on the disk ; the adhesion is strongest 

 when the point is negative. The strain required to make the paper slide 



* Proc. Eoy. Soc. vol. xxiii. p. 356. 

 t Proc. Eoy. Soc. vol. xxiy. p. 169. 



