1890.] Effect of Tension upon Magnetic Changes of Length. 469 



not such close approximations to orifices in truly thin plates as to 

 warrant the acceptance of this result without apparatus of more 

 elaborate construction. Moreover, there is a possibility of slight leak 

 in the grooves of the shutters, which ought not to be disregarded. 



The observations are, however, sufficient to show that a properly 

 constructed apparatus is capable of making measurements of the 

 effective areas of orifices with a very considerable degree of precision. 

 It is well known that if the orifice be not an aperture in a thin plate, 

 but in the form of a tube, straight or bent, the flow through the 

 orifice can be represented by an equation of the same form as if the 

 orifice were a thin plate aperture, viz. : — 



H = RV2, 



but in the case of a more complicated orifice R cannot be so easily 

 calculated from the dimensions ; the value of R might, however, 

 be determined experimentally for an orifice of any shape and dimen- 

 sions by a pneumatic bridge of suitable size, and the result might be 

 expressed, as M. Murgue suggests for the case of mines in the 

 work already referred to, by stating the area of the thin plate 

 orifice to which the given orifice is equivalent. The comparison of 

 calculated values of R with observed values obtained by a pneumatic 

 bridge would enable us to determine a number of pneumatic con- 

 stants that are at present only comparatively roughly ascertained, 

 such, for instance, as the coefficient of air friction in tubes of different 

 diameters, the constants of different forms of orifice, the effect of 

 beads and elbows in pipes, and of gauze or gratings covering an 

 orifice. And it would not, I think, be difficult to arrange the ap- 

 paratus in such a way as to determine the law of resistance of a disc 

 to the passage of air and its variation with velocity. The velocity 

 can be increased to any extent that may be necessary by using a 

 centrifugal fan to produce the head instead of the gas burner. 



I am intending, if possible, to have my present apparatus altered 

 in some of its details, so that the orifices may be more definitely 

 expressed in terms of thin plate apertures, and then to use it for the 

 determination of some of the pneumatic constants I have referred to. 



II. " On the Effect of Tension upon Magnetic Changes of 

 Length in Wires of Iron, Nickel, and Cobalt." By Shelford 

 Bid well, M.A., F.R.S. Received April 8, 1890. 



Preliminary. 



A former communication to the Royal Society (' Roy. Soc. Proc.,' 

 No. 243, 1886, p. 257) contains an account of some experiments 

 relating to the magnetic extensions and contractions of iron wires 



