ELECTRICITY. 



281 



the deflections were very feeble, and the chemical 

 action on the plates not perceptible; while with 

 others, such as nitric and chloric acid, solutions of 

 the chlorides of strontium, sodium, potassium, and 

 ammonium, and of carbonate, acid carbonate, and 

 cyanide of potassium, the deflections were consid- 

 erable, and the chemical action distinct, and in some 

 cases strong. In none of the liquids (except hydro- 

 bromic and chromic acids) did the hot plates appear 

 to be less stained or corroded than the cold one ; 

 probably, in all cases, it was the most corroded, 

 although in some cases the corrosion was iiot per- 

 ceptible. 



The amount of deflection was not always propor- 

 tionate to the amount of chemical action ; for instance, 

 with solutions of chloride of copper, and iodate of 

 potassium, there was considerable corrosion, but 

 only feeble currents, probably because the plates 

 became covered with a badly-conducting film, while 

 with hydrochloric acid, chloride of cobalt, chloride 

 of manganese, and nitrate of potassium, the reverse 

 occurred. 



The electric currents produced by the direct in- 

 fluence of unequal temperature or friction of platinum 

 or copper electrodes, in conducting liquids which do 

 not act chemically upon those metals, have their 

 origin in temporary changes of cohesion of the layers 

 of metal and liquid whicn are in immediate and mu- 

 tual contact, and may be considered as a very deli- 

 cate test of the kind and amount of temporary mo- 

 lecular movements produced by those causes. 



Resistance and Temperature. Mr. C. "W. 

 Siemens read a paper before the Royal Society, 

 in April, recounting his investigations into the 

 ratio of increase of resistance in metallic con- 

 ductors with increase of temperature. He 

 suggested various forms of apparatus for meas- 

 uring with great accuracy the temperature at 

 distant and inaccessible places, including the 

 interior of furnaces where metallurgical or 

 other smelting operations are going on. 



In measuring temperatures not exceeding 

 100 Cent., the instrument is so arranged that 

 two similar coils are connected by a light cable 

 containing three insulated wires. One of these 

 coils, the "thermometer-coil," being carefully 

 protected against moisture, may be lowered 

 into the sea, or buried in the ground, or fixed 

 at any elevated or inaccessible place whose 

 temperature has to be recorded from time to 

 time; while the other, or "comparison-coil," 

 is plunged into a test-bath, whose temperature 

 is raised or lowered by the addition of hot or 

 cold water, or of refrigerated solutions, until 

 an electrical balance is established between 

 the resistances of the two coils, as indicated 

 by a galvanoscope, or by a differential voltam- 

 eter, which balance implies an identity of tem- 

 perature at the two coils. The temperature of 

 the test-solution is thereupon measured by 

 means of a delicate mercury-thermometer, 

 which at the same time tells the temperature 

 at the distant place. 



By another arrangement the comparison- 

 coil is dispensed with, and the resistance of 

 the thermometer-coil, which is a known quan- 

 tity at zero temperature, is measured by a dif- 

 ferential voltameter, and the temperature cor- 

 responding to the indications of the instrument 

 is found in a table, prepared for this purpose, 

 in order to save all calculation. 



In measuring furnace-temperatures the plat- 

 inum wire, constituting the pyrometer, is 

 wound upon a small cylinder of porcelain con- 

 tained in a closed tube of iron or platinum, 

 which is exposed to the heat to be measured. 

 If the heat does not exceed a full red heat, or, 

 say, 1000 0., the protected wire may be left 

 permanently in the stove or furnace whose 

 temperature has to be recorded from time to 

 time ; but, in measuring temperatures exceed- 

 ing 1000 C., the tube is only exposed during 

 a measured interval of, say, three minutes, to 

 the heat, which time suffices for the thin pro- 

 tecting casing and the wire immediately ex- 

 posed to its heated sides to acquire within a 

 determinable limit the temperature to be meas- 

 ured, but is not sufficient to soften the porce- 

 lain cylinder upon which the wire is wound. 

 In this way temperatures exceeding the weld- 

 ing-point of iron, and approaching the melting- 

 point of platinum, can be measured by the 

 same instrument by which slight variations at 

 ordinary temperatures are told. A thermo- 

 metric scale is thus obtained, embracing, with- 

 out a break, the entire range. 



The leading wires between the thermomet- 

 ric coil and the measuring instrument, which 

 may be under certain circumstances several 

 miles in length, would exercise a considerable 

 disturbing influence if this were not eliminated 

 by means of the third leading wire before men- 

 tioned, which is common to both branches of 

 the measuring instrument. 



Another source of error in the electrical 

 pyrometer would arise through the porcelain 

 cylinder, upon which the wire is wound, be- 

 coming conductive at very elevated tempera- 

 tures; but it is shown that the error arising 

 through this source is not of serious import. 



Magnetic Spectra. Mr. Alfred M. Mayer 

 recommends the annexed plan to preserve the 

 magnetic spectra or the figures produced by 

 iron filings, when these are set in momentary 

 vibrations on a surface placed over a magnet. 

 The ordinary process of "fixing" them, so 

 that they can be exhibited and studied, is to 

 press upon them a sheet of paper, coated with 

 mucilage, to which the filings adhere ; but in 

 the operation of the transfer many particles 

 are disarranged from their position, and the 

 figures yet more distorted by the shrinking of 

 the wet paper. Mr. Mayer's process is thus 

 given : 



My process is as follows : A clean plate of thin 

 glass is coated with a firm film of shellac, by flowing 

 over it a solution of thin substance in alcohol, in the 

 same manner as a photographic plate is coated with 

 collodion. After the plate has remained a day or two 

 in a dry atmosphere, it is placed over the magnet, or 

 magnets, with its ends resting on slips of wood, so 

 that the under surface of the plate just touches the 

 magnet. Fine iron filings, produced by " draw- 

 filing " Norway iron, which has been repeatedly an- 

 nealed, are now sifted uniformly over the film of lao 

 by means of a fine sieve. The spectrum is then pro- 

 duced on vibrating the plate, by letting fall vertically 

 urjon it, at different points, a light piece of copper 

 wire. The plate is now cautiously lifted vertically 



