22(3 Mr. J. W. Peck on the Steady 



in diameter, and placed about 17 cm. above the object to be 

 .r-ray-graphed. [Time of exposure two to three minutes.] 



Another way (fig. 3) of exciting the effects is to place a 

 flat spiral of wire S above the bulb, the spiral forming a 

 portion of the secondary circuit of the Tesla inductor, pro- 

 vided with a spark-gap having spherical terminals. 



If a long exhausted tube (fig. 4) (30 cm. long, 1*5 cm. 

 diam.) be excited in any of the ways mentioned, at A, and a 

 conductor be placed close to or in contact with the tube at 

 B, then the #-ray effect will be found opposite to B, acting 

 approximately in the direction G. 



The apparatus here described is now being reconstructed 

 so that much greater pressures may be used in the spark-gap 

 vessel, but even with the pressures already reached the dis- 

 charge from the secondary of the Tesla-coil is enormously 

 increased. 

 June 18, 1902. 



P.S. — I also find that the high-pressure spark-gap, when 

 placed in the secondary of an ordinary induction-coil, increases 

 its effects considerably when used as a generator of Hertz 

 waves. A pressure of one atmosphere alters the length of a 

 spark between brass balls from 2*5 cm. to 0*75 cm. 

 July 12, 1902. 



XXV. The Steady Temperatures of a Thin Rod. By James 

 W. Peck, M.A., Arnott and Thomson Demonstrator in 

 Physios in the University of Glasgow *, 

 TT1HE well-known Fourier problem referred to in this paper 

 JL may be briefly stated as follows : — A homogeneous rod 

 of small cross-section and great length has one end maintained 

 at a constant temperature greater than that of the medium in 

 which the rod is placed. By lateral radiation into this medium 

 (also kept at a constant temperature) the rod acquires a steady 

 distribution of temperature, diminishing as we go along the 

 bar from the heated end. If V denote the temperature of 

 the hot end (that of the medium being taken as zero), e the 

 emissivity (condiicibilite exterieure) of the surface of the bar, 

 k the conductivity of the material, s the cross-section (uni- 

 form), p the perimeter, then v the temperature at the distance 

 x from the hot end is given by f 



»- v -*(-V9 (1) 



* Communicated by Prof. A. Gray, F.R.S. 



f Fourier, TheorieAnalytique de la Chaleur^.bS, Darboux's edition. 



