Signalling through space without wires : being a description of the work of Hertz & his successors . eir natureare not the only elements to be considered, for grains of leadof the same size, but coming from different quarters, offer atthe same temperature great differences in resistance (20,000 h2 100 SIGNALLING WITHOUT WIRES. to 500,000 ohms). Extremely fine metallic powder, as a ruleoffers almost perfect resistance to the passage of a if we take a sufficiently short column and exert asufficiently great pressure a point is soon reached when theelectrical influence will effect a su
Signalling through space without wires : being a description of the work of Hertz & his successors . eir natureare not the only elements to be considered, for grains of leadof the same size, but coming from different quarters, offer atthe same temperature great differences in resistance (20,000 h2 100 SIGNALLING WITHOUT WIRES. to 500,000 ohms). Extremely fine metallic powder, as a ruleoffers almost perfect resistance to the passage of a if we take a sufficiently short column and exert asufficiently great pressure a point is soon reached when theelectrical influence will effect a sudden increase in the conduc-tivity. Thus, a layer of copper reduced by hydrogen, whichdoes not become conducting under the influence of the electricspark or otherwise, will become so on being submitted to apressure of 500 grammes to the square centimetre (7ib. persquare inch). Instead of using pressure, I employed as a con-ductor in some experiments a very fine coating of powderedcopper spread on a sheet of unpolished glass or ebonite E(Fig. 55), seven centimetres long and two centimetres Fig. 55. A layer of this kind, polished with a burnisher, has a verjvariable resistance. With a little care one can prepare sheetswhich are more or less sensitive to electrical action. Metal powder or metal filings are not the only sensitivesubstances, as powdered galena, which is slightly conductingunder pressure, conducts much better after having beensubmitted to electrical influence. Powdered bincxide ofmaganese is not very sensitive unless mixed with powderedantimony and compressed. Making use of the A arrangement, with very short sparksat S (Fig. 35), the phenomenon of increased conductivity canbe observed with platinised and silvered glass, also with glasscovered with gold, silver and aluminium foil. Some of the BRANLYS EXPERIMENTS. 101 mixtures employed had the consistency of paste. These weremixtures of colza oil and iron, or antimony filings, and ofether or petroleum and alumi
Size: 1639px × 1524px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookidsi, booksubjectelectricity
