1871.] 



Electricity, 



119 



same time may be traversing the wire from end to end, and even here the 

 limiting power of transmission of the wire is not reached. Two sets of wave- 

 signals may pass through the wire independently of each other, if a sufficient 

 difference be made in the periods of the two sets of waves ; and Mr. Frederick 

 Varley, who made much of the apparatus, informs the writer that he has very 

 readily sent three messages from end to end of the same length of wire at the 

 same time. 



Where the distance between any two singing instruments exceeds 100 or 150 

 miles, the difficulties in signalling with them are increased ; transmitting 

 intermediate stations may therefore be necessary for very long distances. The 

 instruments have also to be tried on ordinary telegraphic lines, to find out their 

 real practical value. Whether the commercial value of Mr. Varley's plan of 

 telegraphing be very great or very small, it is certainly one of the cleverest 

 scientific inventions of the present age. 



Siemens's Electrical Pyrometer. — Mr. C. W. Siemens, C.E., F.R.S., of Great 



George Street, Westminster, has recently invented an ingenious pyrometer, the 

 principle of which is, that as the electrical conductivity of platinum, iron, and 

 other metals decreases as they rise in temperature, their increase of resistance 

 to the passage of the current is a measure of the heat to which the metals are 

 subjected. 



The principle of construction may be explained by the aid of Fig. 23, in which 

 F A B is a tube of pipe-clay, and the length between the projections, a and b, has 

 a screw-shaped spiral groove cut on its outer surface ; the 

 Fig. 23. length of this part of the tube is about 3 inches. A spiral of 

 p fine platinum wire lies in the groove, each turn of the platinum 



spiral being thus protected from lying in contact with its 

 neighbour by the projecting edges of the groove, by which 

 plan of insulation the current is forced to pass through the 

 whole length of the fine wire, d is a little platinum clam, 

 connected with one pole of the battery, and the position of 

 this clam on the spiral regulates the length of platinum wire 

 through which the current shall pass. By this plan of adjust- 

 ment, all the pyrometers constructed by Mr. Siemens are made 

 to agree with each other. 



At f, the ends of the thki platinum wire are connected with 

 very thick platinum wire, and higher up, near e, where the 

 heat of the furnace is less felt, the thick platinum wires are 

 connected with thick copper wires, shown at p ; from e to f, 

 these connecting wires are protected by clay pipes, as shown 

 in the cut. 



When this arrangement has to be used, the whole of it 

 is dropped into a thick metal pipe made of iron, copper, or 

 platinum, according to the heat of the furnace to be tested. 

 The lower end of this outer pipe is shown at k m, and when 

 it is used, the spiral A b, lies inside it at n m. At r there is 

 a very thick collar of metal in which the heat accumulates, 

 and this prevents the cooling action of the length k r (most 

 of which does not enter the furnace), from interfering with 

 the accuracy of the indications. The ends of the wires, p, 

 are connected with suitable and very delicate electrical 

 apparatus, by which the increasing electrical resistance of the 

 hot spiral is measured. 



To measure low temperatures on the same principle, 

 several miles of insulated iron wire are enclosed in a tube 

 containing dry air, and hermetically sealed. This instrument 

 is very useful to measure underground temperatures, as the 

 indicating part of the apparatus may be far away from 

 the thermometric portion. Dr. Carpenter used this appa- 

 ratus, with some modifications, in ascertaining deep sea 

 temperatures. 



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