for Calibrating Thermometers. 295 



process is described, which, although involving little that is 

 novel, has not to my knowledge been used before. 



First, however, it is necessary to recall to the attention 

 of observers the fact that, without calibration-correction, the 

 readings of a thermometer having a scale of equal linear parts 

 cannot be relied upon within one or more divisions of this 

 scale, and that thermometer-makers, knowing this, almost 

 universally space the graduation upon the tube to correspond 

 more or less closely with the shape of the bore, as determined 

 by previous calibration, or by comparison with a standard (!) 

 instrument. This practice is much more general than is ordi- 

 narily supposed, and has an important bearing upon the accu- 

 racy of the work done with such instruments. For the scale 

 thus made is merely approximate, the dividing-engine or other 

 tool being usually changed only at such intervals as to make 

 the average error less than some specified amount. An inspec- 

 tion of these conditions will show that the calibration of such 

 a tube and scale can be only approximate, except with correc- 

 tions for the inequalities of the spacing, involving an amount 

 of labour disproportionate to the result obtained. The best 

 makers, such as Fastre, Baudin, and others, have produced 

 satisfactory thermometers graduated to equal volumes ; but 

 even these are not as reliable as instruments of less cost with 

 a scale of equal linear parts, say of millimetres, supplemented 

 by a calibration by the observer. The best form of tube for 

 almost all work is one backed with white enamel, with an 

 inverted pear-shaped bulb at the upper end of the capillary 

 (a very important feature), and with a scale of equal arbitrary 

 linear parts (0*7 to 1 millim. is a suitable length for estimation 

 of tenths) or of approximate degrees, for convenience, etched 

 or engraved upon it. 



Without reviewing here the methods proposed by various 

 writers, it may be said that it has been the general plan to 

 select beforehand upon the scale two points between which to 

 make the calibration, this space being the "calibration unit," 

 the errors of these points being, of course, zero. This plan 

 has led to unnecessary complexity. Such an assumption is no 

 more requisite in calibration after a scale has been put upon 

 the tube, than in calibrating by the dividing-engine or micro- 

 meter before making the scale. It is obvious that the selec- 

 tion of these points is wholly arbitrary, and, if used at all, one 

 or both of them may, if desirable, be chosen after the observa- 

 tions with the calibratlng-thread have been made. The choice 

 should be made with a view to facilitating the work. Hence 

 the use of the observed freezing- and boiling-points, upon 

 which some methods are based, is most undesirable. 



