404 



ELECTKICITY. 



standard be prepared, and of such substance and 

 form as to insure the most absolute permanency. 

 The aim should be to make this standard cor- 

 respond to a current force equal to 10, 000, 000,- 

 000 times the value given by the quotient of 1 

 metre by 1 second of time, that is, lO 10 ^,^.-; 

 and to this it should approximate as nearly as 

 possible. Such a unit would not differ^ more 

 than .03 for Dr. Siemens' mercury unit. It 

 should not be called an absolute unit, but sim- 

 ply the " unit of 1862 ;" and it should not be 

 constructed at all until a very close approxima- 

 tion to the absolute value was supposed to 

 have been attained, and great permanency in 

 the material standard secured. Then, as the 

 advance of science showed more and more truly 

 the actual deviation of this from the true unit 

 intended, corrections could be made by experi- 

 menters in their results when required. The 

 material standard itself, however, should under 

 no circumstances be altered in substance or 

 definition. 



Influence of Temperature on the Conducting 

 Power of Metals. Matthiessen and Van Bose 

 presented before the Royal Society a paper on 

 this subject, Jan. 16, 1862. They find that, 

 contrary to what has been stated by Becquerel, 

 Siemens, and others, the conducting power or 

 the resistance of a metal for an electric current, 

 does not increase or decrease in a direct ratio 

 to the temperature, but in a ratio much more 

 complex. All pure metals in the solid state, 

 however, vary in conducting power to the same 

 extent between the temperatures of 32 and 212 

 F. Wires of the same metal behave differently 

 after being kept for some days heated to the 

 same degree. Metalloids generally conduct bet- 

 ter when heated than when cold ; this is true 

 of selenium, gas coke, graphite, and the gases. 

 Mechanical Effects of Powerful Tension. M. 

 Faye exhibited before the Academy of Sciences, 

 Paris, an experiment in which two plates of 

 crown glass, respectively nearly 1 i and 2| inches 

 thick, were completely pierced through by the 

 electric spark of the great induction machine 

 recently constructed by M. Ruhmkorff. The 

 path left by the spark is seen to consist of a 

 white and opaque fillet, extremely slender, its 

 whole length presenting bright places at slight 

 distances, and taking different directions in the 

 manner of the parts of a spiral. 'It shows no 

 metallic deposit. In the thicker plate, at a 

 depth of about 3, the track bifurcates ; and near 

 to the opposite face, it subdivides into many 

 and more direct fillets. During the experiment, 

 Ruhmkorff demonstrated by the appearance of 

 Haidinger's colored rings, that the passage of 

 the spark was accompanied by an energetic com- 

 pression of the substance of the glass ; but no 

 trace of fusion appeared about the course taken 

 by it. M. Faye thought nevertheless that, by 

 passing the spark of this machine through some 

 pulverulent substance somewhat more fusible 

 than crown glass, true fulgurites coull be pro- 

 duced. 



Study of the Electric Spark ly Aid of Pho- 

 tography. Prof. O. N. Rood, of Troy Univer- 

 sity, N. Y., has been, by aid of a new and very 

 neat method devised by him, studying the form 

 and characteristics of the electrical spark. The 

 photographic images of the electric spark hith- 

 erto obtained by Prof. B. Silliman, jr., Prof. 

 W. B. Rogers, Feddersen and others, have 

 been taken from a position at right angles to 

 the course of the spark, thus furnishing a side 

 view of it ; and they have usually required a 

 prolonged exposure of the sensitive ' surface to 

 the light, in some instances from 3 to 6 minutes. 

 Prof. Rood's method is that of receiving the 

 spark directly upon a sensitive or properly 

 prepared surface, and subsequently developing 

 the figure produced, in the manner of photog- 

 raphy. In this way he secures the figure cor- 

 responding to a single discharge of ordinary or 

 frictional electricity, the brief duration of 

 which has forbidden its being photographed by 

 previous methods. His plan is an applica- 

 tion, first, of Becquerel's discovery that paper 

 coated with bromide of silver is sensitive to the 

 electric spark ; and, secondly, of his own obser- 

 vation that, in instantaneous photography, the 

 portions of the sensitive surface immediately 

 contiguous to those acted on by the strong 

 lights, still remained quite unaffected by the 

 exposure. Upon testing the fact in case of a 

 single electric spark, he found that an intense 

 and sharply defined image, full of delicate de- 

 tails, was here also the result. The question 

 whether these images were due to direct action 

 of the electricity itself, or to the agency of the 

 light evolved, appeared to be settled by certain 

 experiments, especially by the fact that when 

 the spark was received on a thin glass plate, 

 placed over another coated with sensitive collo- 

 dion, the electric image could be developed, 

 though less sharply defined, on the second 

 plate; but when the first or thin glass was 

 blackened, no image on the second resulted, 

 from one or a number of discharges. The form 

 of apparatus given for these experiments is 

 simple ; the collodion recommended is that 

 suitable for ambrotyping, and when freshly 

 prepared. 



The general form of the positive electrical 

 Bpark ascertained, is a combination of two 

 figures : a star and one or more rings, all hav- 

 ing nearly a common centre. The rings are 

 usually quite within the limits of the star, 

 sometimes one of them without it ; and when 

 two or more rings appear, they are successively 

 darker toward the centre. The marked differ- 

 ences in form of the two components, and the 

 fact that the annular form is characteristic of 

 the electric brush, seem to indicate that each 

 simple spark consists of two or more successive 

 discharges of varying intensity. When, owing 

 to distance, or to the use of a pointed wire, the 

 partial sparks become more uniformly blended, 

 the electrical " brush " is the result; and^the 

 figures confirm the general view of electricians 

 on this point, by showing how the former passes 



