PHYSICS, PROGRESS OF, IN 1890. 



717 



in the solar spectrum. 5. The rays must be ab- 

 sorbed in order to act. 6. All metals and some 

 aniline dyes are thus affected ; water is not. 7. 

 The effect requires no appreciable time. 8. The 

 discharge is proportional to the energy of the 

 rays and to the surface illuminated. 9. Its mag- 

 nitude depends on the density, at first increas- 

 ing more rapidly than the density, but after- 

 ward more slowly. 10. Two plates between 

 which there exists a contact difference of poten- 

 tial act as a voltaic element when the negative 

 one is illuminated. 11. We can consider this 

 discharge as an electric current. The air plays 

 the part of a bad conductor and Ohm's law is 

 not followed. 12. The actino-electric action in- 

 creases with the temperature. 



The phenomena are seen only in gases, and are 

 due to a kind of convection 'current, but the 

 first step of excitation remains an enigma, though 

 the analogy with Geissler and Crookes tubes is 

 striking. The ultra-violet radiations reduce to 

 the same potential a plate and netting (see " An- 

 nual Cyclopaedia," 1889, page 695), and Righi 

 utilizes this fact in measuring the potential differ- 

 ence of contact of metals. He connects the net- 

 ting permanently to earth, notes the deflection 

 of an electrometer connected with the plate, 

 then connects the electrometer to earth and 

 turns on the light ; the difference of the two 

 electrometer readings gives the quantity sought. 

 Prof. G. M. Minchin has constructed what he 

 calls "photo-electric impulsion cells." (''Nature," 

 May 22). Two metal plates are immersed in al- 

 cohol in a glass tube. One of the plates has pre- 

 viously been prepared in a way which has not 

 yet been described by the inventor. Light caus- 

 es a deflection of a galvanometer that is joined 

 to the plates ; but tapping on the base of the 

 cell renders tt insensitive. A second tap restores 

 the sensitiveness, and so on. A spark from a 

 Voss machine that is not connected with the 

 cell has the same result. Prof. Minchin regards 

 the phenomena as due to the formation of an 

 "oscillating layer" at the surface of the plate. 

 Details and full conclusions are yet to appear. 



Thermo-electricity. Chassagny and Abra- 

 ham (Paris Academy of Sciences, Sept. 29) find 

 that the variation in the electromotive force pro- 

 duced by heating the poles of a copper-iron 

 couple is practically constant between and 

 100 C. They suggest that some such arrange- 

 ment be used as a standard of electromotive 

 force. The same experimenters (Oct. 27), in ver- 

 ifying the law E(AC)=E(AB) + E(BC), where 

 E(AC) means the contact electromotive force be- 

 tween the metals A and C find the following: 



Liebisch, of Gottingen, has investigated 

 thermo-electric currents in crystals. He finds 

 that in a rectangular parallellopiped of homo- 

 geneous conductivity cut from a triclinic crystal, 

 and imbedded in homogeneous normal metal, 

 the thermo-electric force in the direction of the 

 steepest temperature gradient is always repre- 

 sented by the squared reciprocal of the parallel 

 radius vector of a certain ellipsoid. Dr. de 



Coudres has detected a thermo-electric tension 

 between compressed and uncompressed m.-ivurv 

 1 he compression was effected both hydraulic-ally 

 and by the weight of the liquid itsdi. 



Disruptive Discharge. A. Schneider has in- 

 vestigated the disruptive discharge through 

 gases, and finds: 1, that the explosive distant. 

 is a function of the curvature ; 2, that the break- 

 ing stress diminishes for increasing distances 

 between two planes ; and 3, that the explosive 

 stress reaches a minimum with increasing dis- 

 tance, and then increases again. The increase 

 of stress at small distances is due to the inequal- 

 ity of the field along the surface of the con- 

 ductor. In the case of planes the inequality 

 lies between the back and front of the surface. 

 Possibly the density of a curved layer may de- 

 pend also on an inequality of the field along the 

 normal to the surface. Dr. S. P. Thompson 

 (London Physical Society, Jan. 17) has experi- 

 mented on electric "splashes" a modification 

 of the well-known Lichtenberg figures. The 

 nature of the dielectric plate does not affect the 

 figures, nor does the nature of the powder, but 

 a mixture of sublimed sulphur and lycopodium 

 shows them best. He used a large and highly 

 polished knob. In the negative figures nebulous 

 patches occur, which he attributes to electric 

 winds. If the knob be brought near the plate 

 without touching it, a peculiar figure like a 

 splash results. A positive splash consists of 

 short radiating lines, and a negative one of more 

 or less rounded spots. In the dark, a splash is 

 a bundle of small sparks branching outward. 

 Similar results have been obtained by others by 

 discharges on photographic plates. J. Stefan 

 (Vienna, " Berichte," June 7) says that an oscil- 

 latory discharge shows more markedly than any 

 other the properties of electric inertia. The 

 phenomena resemble those of a liquid oscillating 

 in connecting tubes. The energy that corre- 

 sponds to difference in height is changed to 

 vis viva and back again. The question is, What 

 is this vis viva in electricity which is capable of 

 being changed back into difference of potential f 

 In the special case in which a wire passes around 

 an iron core the nature of the greatest part is 

 known. It is the magnetic energy which has 

 accumulated in the iron. Even when there is 

 no iron, then, the energy may be conceived of as 

 magnetic. 



Electric Convection. Prof. H. A. Rowland 

 and C. T. Hutchinson have repeated the former's 

 Berlin experiment on the magnetic effect of a 

 statical charge in motion, and find the deviation 

 of the needle proportional to the amount of elec- 

 tricity passing per second, as with voltaic cur- 

 rents. F. Himstedt has repeated the same ex- 

 periment, and confirms its results, except that 

 he finds no difference in changing the direction 

 of rotation. 



The Electric Arc.E. Villari ("Accademia 

 dei Lincei," v, 730) finds that Jhe arc is far 

 shorter in hydrogen than in carbon dioxide, and 

 in the latter than in air. The ratio is 3*9 : 

 7'4 : 8-5. In nitrogen, with an ascending cur- 

 rent, the arc was 7 times as long as in hydrogen, 

 and with a descending current it was 25'7 times 

 as long. 



Resistance. M. Curie (" Annales de chimie et 

 de physique") has investigated the inductive 



