28 REPORT—1874., 
have at first a simple flowing off of the electricity, occupying about the period of 
one oscillation; but, when the resistance becomes greater still, the discharge 
occupies a longer time. This is the continuous discharge. The limiting resistance 
in its turn follows special laws*; it is inversely proportional to the number 
of jars (capacity), and increases slowly with the length of the discharging circuit 
(potential of the current). 
The correctness of the author's interpretation of the electrical photographs, and 
of similar experiments since made by others, as well as of the conclusions deduced 
from it—that in almost all cases when a Leyden jar is discharged through a metallic 
conductor an oscillatory discharge takes place, and that the usual mode of representing 
the motion of electricity during the discharge (which we meet with, for instance, in 
Riess’s ‘Die Lehre von der Reibungselectricitat’) is incorrect-—follows, 
Ist. From the agreement with theory. Sir William Thomson} and Professor 
Kirchhoff { had previously shown, by theoretical considerations, that under some 
circumstances an electric discharge must exhibit such oscillations. Subsequently 
Kirchhoff submitted the author’s observations to a numerical calculation§, and, 
disregarding a constant factor, found them to agree completely. Moreover the 
author has himself examined || theoretically the laws of the strength of the currents 
in divided circuits in the case of an oscillating discharge, and has discovered their 
agreement with certain remarkable observations by Knochenhauer on the electrical 
air-thermometer, as well as their relation to his observations of oscillation. 
2nd. From an experimental point of view, the interpretation that has been given 
is supported by a variety of other investigations. Without reckoning the experi- 
ments of Priestley, Savary, Riess, and others, which were made some of them a 
long time ago and only doubtfully indicate this explanation, the following expe- 
rimental results may be mentioned :—the proof given by Paalzow** that in some 
discharges there is a current in two opposite directions; the negative residual 
charge obtained by Von Oettingertt (though not under ordinary conditions) with a 
positively charged Leyden jar; and, lastly, the sixteenfold increase of the deflection 
of a galvanometer observed by the author {+ when the oscillatory discharge was sent 
through its coils interposed between a Gaugain’s double electrical valve. Time did 
not allow of mentioning other experiments and observations of the author which 
bear on the same point, and are to be found in various places in Poggendorff’s 
‘ Annalen.’ ! 
The question has been raised whether the photographically represented oscillations 
might not coincide with the discontinuous discharge observed by Wiedemann and 
Ruhlmann §§. The author contradicts this suggestion, and adds that he had ob- 
served |||| similar partial discharges before he discovered the oscillations of the 
discharge of a Leyden jar. The discontinuous discharge, however, only occurs 
with the highest resistances, such as those offered by capillary columns of distilled 
water, and when the discharging circuit is well insulated; and successive partial 
discharges differ from the oscillations in following each other at continually greater 
and greater intervals instead of at a constant distance. 
Geometrical Jlustrations of Ohm’s Law. By Professor G. C. Foster, F.R.S. 
The object of this paper is to point out an easy method of deducing the permanent 
resistance and electromotive force of a galvanic circuit from two observations with 
a tangent- or sine-galvanometer, without using trigonometrical tables. 
I. Observations with a tangent-galvanometer.—A battery, connected in simple 
* Pogg. ‘Annalen,’ exii. 452 et seq. t Phil. Mag. [4], v. 393 e¢ seg. 
$ Pogg. ‘Annalen,’ c. 193. § Pogg. ‘Annalen,’ exxi. 551 e¢ seg. 
|| Zbid. exxx. 439 et seq. 
{| ‘The observations referred to are to be found scattered through Knochenhauer’s various 
papers, published for the most part in Poggendorff’s ‘ Annalen.’ 
**. Paalzow allowed the discharge-current to pass through a Geissler’s tube aboye the 
poles of a powerful electromagnet (Pogg. Annalen, cxii. 572). 
tt Pogg. ‘Annalen,’ exv. 513 e¢ seq. tt Ibid. exv. 336 et seq. 
§§ Zbzd. exly, 235 et seg. || || Zo2d. citi. 69 e¢ seg. 
