May 16, 1872] 



NATURE 



49 



meteorological purposes, or for measuring the temperature 

 of the sea continuously in attaching such a coil to the 

 mariner's sounding lead. An error would in such 

 cases arise, however, through the uncertainty of the 

 lesistance of long leading wires, if a complete remedy 

 of error from such a source had not suggested 

 itself. This consists in uniting three separate insulated 

 leading wires into a cable by which the distant coil is con- 

 nected with the measuring instrument. One galvanic 

 circuit passes from the batteiy through one of the leading 

 wires, through the distant spiral, and back again through 

 the second leading wire to the differential galvanometer 

 and the battery, and the second passes from the same 



battery through the near coil, and through the third lead- 

 ing wire up to the distant coil without traversing the same, 

 and back again through the second leading wire to the 

 galvanometer and battery. Thus both galvanic circuits 

 comprise the leading wires up to the distant coil, and all 

 variations of resistance by temperature to which the lead- 

 ing wires may be subjected affect both sides of the balance 

 equally. In constructing coils for measuring deep-sea 

 temperatui'es a large quantity of insulated copper or iron 

 wire is wound upon a metallic tube open at both ends to 

 admit the sea-water freely in order to impart its tempera- 

 ture to the innermost layers of the insulated wire. The 

 coil of wire is protected e.^itcrnally by di'awing a tube of 



vulcanised india-rubber over it, which in its turn is bound 

 round by a close spiral layer of copper wire, whereby the 

 sea-water is effectually excluded fi'om the sensitive coil. 

 Cy these arrangements the temperature of distant or 

 otherwise inaccessible places can be accurately ascertained ; 

 but the method is limited to the range of temperature 

 which can be obtained and measured in the comparison 

 bath. In order to realise a pyrometer by electrical resist- 

 ance, it is necessary to rely upon the absolute measurement 

 of the electrical resistance of a coil of wire which must 

 be made to resist intense heats without deteriorating j 



through fusion or oxidation. Platinum is the only suitable 

 metal for such an application, but even platinum wire 

 deteriorates if exposed to the direct action of the flame of 

 a furnace, and requires an external protection. The 

 platinum wire used has, moreover, to be insulated and 

 supported by a material which is not fused or rendered 

 conductive at intense heats, and the disturbing influence 

 of leading wires had in this case also to be neutralised. 

 These various conditions are very fully realised by the 

 arrangement represented on the preceding diagram, Fig. 4. 

 Thin platinum wire is coiled upon a cylinder of hard- 



baked porcelain, upon the surface of which a double- 

 threaded helical groove is formed for its reception, so as to 

 ]>: event contact between the coils of wire. The porcelain 

 C)linderispiercedtwice!ongiludinally for the passage of two 

 thick platinum leading wires, which areconnected to the thin 

 sp'ral wireat the end. In the upper portion of the porcelain 

 cylinder the two spiral wires are formed into a longitu- 

 dinal loop, and are connected crossways by means of a 

 platinum binding screw, which admits of being moved up 

 or down for the purpose of adjustment of the electrical 

 resistance at the zero of Centigrade scale. The porcelain 

 cylinder is provided with projecting rims, which separate 

 the spiral wire from the surrounding protecting tube of 

 platinum, which is joined to a longer tube of wrought iron, 

 serving the purpose of a handle for moving the instrument. 



If the temperatures to be measured do not exceed a 

 moder-ate white heat, or say 1,300^ Centigrade = 2,372° 

 Fah., it suflices to make the lower prolcciing tube'also 

 of wrought iron, to save expense. This lower portion 

 only, up to the conical enlar-gement or boss of iron, is e :- 

 posed to the heat to be measured. Three leading wires 

 of insulated copper united into a light cable connect the 

 pyrometer with the measuring instrument, which may be 

 at a distance of some hundred yards from the same. They 

 are connected by means of binding screws at the end of 

 the lube to three thick platinum wires passing down the 

 tube to the spiral of thin p'atinum wire. Hei-e two of the 

 leading wires are united, whei-eas the third traverses the 

 spiral, and joins itself likewise to one of the two former, 

 which forms the return wire for two electrical c'rcuits, the 



