282 



ELECTRICITY. 



watchmen) ; when the knob is pressed, the 

 paper is punctured by a point, thus recording 

 the very minute the watchman was at a given 

 post. 



Electro- Chemical Copy ing- Press. An elec- 

 tro-chemical copying-press has been invented 

 by Signor Zuccator, of Padua, having for its 

 object the reduplication of any writing and 

 printing to any desired extent. The upper bed 

 of the press consists of a plate of copper, and 

 the lower bed of plate of copper tinned, both 

 resting on mahogany beds. These plates are 

 placed in the ordinary way in the circuit of a 

 battery, so that when brought into close con- 

 tact the circuit is completed, and a current 

 established over the whole of the surfaces. By 

 the aid of a varnish (an insulating medium) 

 applied to a steel plate, and removable by the 

 action of a " style " by which the writing, etc., 

 is executed, the electric current is confined to 

 those portions any of which are so denuded 

 of the insulating protection; and then it is 

 made to leave record of its passage by its con- 

 tinued action on the steel plate, and sheets of 

 copying-paper specially prepared and damp- 

 ened by a solution of prussiate of potash. The 

 electrolytic action causes the formation of ferro- 

 prussiate, known as Prussian blue, producing 

 a perfect fac-simile of the original design 

 wrought on the varnished surface of the plate. 

 The battery employed consists of a single cell, 

 with zinc and carbon elements in an actuating 

 solution of bichromate of potash and sulphuric 

 acid ; and its positive and negative poles are 

 connected in the usual way, by spiral coils of 

 insulated wire, with the upper and lower beds 

 of the copying-press. The movable steel 

 plates, on which the writing, drawing, or other 

 design, to be copied, is made, has to be thor- 

 oughly cleaned and well and evenly varnished ; 

 care must also be taken, by a firm, steady press- 

 ure on the style, effectually to remove the 

 varnish, leaving the writing, printing, or other 

 pattern, in bright steel on a raised ground of 

 varnish, affording perfect insulation every- 

 where else on the surface. By placing the 

 copying-sheets, efficiently damped with the 

 prussiate solution, in any number from one to 

 five or six, one over the other, superimposed 

 on the prepared plate, a corresponding number 

 of copies can be obtained, and so on, almost 

 ad infinitum. Thus any required number of 

 copies can be produced with perfect facility 

 and ease all being fac-similes of the original. 



Duration of the Electric Spark. A com- 

 mittee of the French Academy have reported 

 favorably on the method adopted by MM. 

 Lucas and Cazin to measure the duration of 

 the electric spark (see ANNUAL CYCLOPAEDIA 

 for 1870). The committee remark that the 

 duration of the spark being determined by the 

 number of coincidences seen by the observer, 

 if the degree of the illumination of the lines 

 were much diminished it is to be feared that 

 the number of coincidences would not dimin- 

 ish equally inconsequence of the enfeeblement 



of the light corresponding to the end of the 

 discharge. And they think it would be of use 

 to ascertain the effect of variations in lumin- 

 ous intensity, as in discharges between elec- 

 trodes of various metals, placed at different 

 distances, in gases at different pressures. The 

 proposers of the method have not been able 

 to make appreciable the duration of a spark 

 from an ordinary machine ; but they found 

 the duration of the discharges of condensers 

 varies with the surface of these, with their 

 arrangement, and the resistance of the circuit. 

 It varies also with the striking distance, the 

 nature of the balls, and the humidity of the 

 air. In general, the duration increases with 

 the condensing surface, and with the distance 

 between the balls, and diminishes with the 

 length of the circuit. The limits of duration 

 given by their observations are four millionths 

 of a second, and eighty millionths of a second 

 with a possible error of one six-millionth of 

 a second. 



Spectrum of Lightning. Lieutenant E. S. 

 Holden, of the West Point Academy, reports 

 to the American Journal of Science his obser- 

 vations on the spectrum of lightning on three 

 occasions, when the display occurred shortly 

 after sunset, and presented rapid successions 

 of sheet-lightning and frequent vivid flashes. 

 He says : 



In the sheet-lightning and in the fainter flashes 

 the green and blue portions of the spectrum were 

 visible, the violet and red cut off ; in the brighter 

 flashes a complete and continuous spectrum appeared 

 and superposed on it bright lines. The red end of 

 this spectrum (of vivid flashes) seemed to be shorter 

 than that of the spectrum of a common gas-jet 

 turned down low, with which it was constantly and 

 almost instantaneously compared, without moving 

 from the place of observation. 



From the sheet-lightning I repeatedly obtained 

 series of bright bands in the green, but the width 

 and intensity of these bright bands continually 

 changed. Of the bright and sharp lines 1 saw but 

 three 1, line in green; 2, line in blue; 3, line in 

 violet (or extreme blue ?). 



These were seen frequently, and sometines those 

 of one flash would be immediately succeeded by 

 those of the following flash, thus giving me a means 

 of assuring myself that the same lines appeared as 

 well in position as in color. 



Electrical Condition of Gas-Flames. The 

 following are some of the results obtained by 

 Mr. Trowbridge, in experiments on the elec- 

 trical condition of gas-flames. The flame 

 operated on was that of a Bun sen burner, its 

 electrical condition being determined by a 

 Thomson's quadrant electrometer. Upon con- 

 necting the testing-plate of one pair of quad- 

 rants of the instrument with the metallic 

 burner, and with the earth, the flame was 

 found to be electrified negatively. 



Experiment 1. Flame twelve centimetres high ; 

 plate at the height of seven centimetres. A nega- 

 tive indication of 130, very steady. 



Experiment 2. A platinum wire, substituted for 

 the plate, and meeting the flame three centimetres 

 above the burner, gave a deflection of 30 in a nega- 

 tive direction. 



Experiment 3. "With the testing plate just above 



