528 Mr. J. S. G. Thomas on 



time as to cause the glass to break up into a number of 

 separate droplets — a tendency on the part of the softened 

 glass, which is explained by well-known considerations 

 relating to surface-tension phenomena. The fusing of the 

 glass on to the wire can also be readily effected by heating 

 the glass and wire cautiously to a predetermined tempera- 

 ture in a small muffle. Thereafter the wire and coating are 

 annealed cautiously, and the wire aged in the usual manner 

 by the passage of a current of about 2 amps, for some hours. 

 An anemometer wire so prepared can be employed in the 

 usual manner in gases such as hydrogen, methane, coal-gas, 

 etc., and yields particularly steady and consistent results. 

 Before passing to a consideration of the results obtained by 

 the use of such an anemometer wire, and of an anemometer 

 wire of the usual type in streams of air (dry and moist), 

 carbon dioxide, hydrogen, nitrogen, and coal-gas, it is of 

 interest to direct attention to a phenomenon differentiating 

 the one type of wire from the other. It has already been 

 remarked (see p. 511) that using an exposed uncovered 

 anemometer wire, the resistance of this exposed w T ire in- 

 creases more rapidly than that of the protected wire of the 

 pair when the current through the wires is increased. The 

 results obtained by the comparison of the resistance of the 

 exposed wire with that of the shielded wire in the case of 

 the bare wire and glass-covered anemometer wire are seen in 

 Plate XIII. fig. 16, the results being given in the case of air 

 for the bare wire anemometer and in the cases where the 

 glass-coated wire is immersed in (1) air, (2) hydrogen, and 

 (3) coal-gas. The wire employed was cut from the same 

 specimen of wire as that employed in anemometer No. 2 (a), 

 and had a diameter of 0'00784 cm. The mean diameter of 

 the glass-coating was O08 cm. It will be seen that in the 

 case of the bare wire surrounded by air, the resistance of 

 the exposed wire increases with current more rapidly than 

 that of the shielded wire with increasing supply of heat to 

 the wire. With the glass-covered exposed wire in air, it is 

 seen that, with increasing supply of heat to the wire, the 

 resistance of the exposed wire decreases compared with that 

 of the shielded wire, the ratio attaining a minimum value 

 and increasing thereafter. With the glass-coated wire sur- 

 rounded by coal-gas, the decrease in the ratio of resistances 

 is still more marked, and attains a minimum value at a rate 

 of supply of heat larger than is the case when the wire is 

 surrounded by air. The result with hydrogen is similar to 

 that in the case of coal-gas, the minimum value of the ratio 

 being, however, smaller and occurring at a still higher value 



