C. GENERAL PHYSICS. 169 



tremely valuable feature when one is obliged to experiment 

 in order to determine the strength of the current necessary 

 for accomplishing a certain work, and one equally valuable 

 to the physicist who may desire to elucidate obscure points 

 in the theory of the machine. The Gramme machine has 

 been still further improved by combining with its peculiar 

 features the construction due to Wild and Ladd, by which 

 an immense magnetic power is developed from a very slight 

 initial movement of magnetism ; by this means an instru- 

 ment has been produced by which the same electric tension 

 is attained with a velocity one half as great as that original- 

 ly necessary. In the course of the numerous improvements 

 that Mr. Gramme has made in his original machine, his latest 

 construction seems to leave nothing to be desired. The 

 number of electro-magnets and of coils is now reduced, from 

 six and twelve respectively, to two and four. The ring is 

 virtually doubled, giving far more facility in the applica- 

 tion of the same machine to very different objects, such as 

 galvanoplasty, lighting, heating, etc. ; in the machines, as 

 originally constructed with a simple ring, each one was only 

 convenient for use for the immediate purpose for which it 

 was designed and proportioned. 13 J5, III., 139. 



THE FRICTION AND THERMAL CONDUCTIVITY OF GASES. 



In a memoir by Messrs. Kundt & Warburg on the friction 

 and thermal conductivity of gases for heat, the authors en- 

 deavor to investigate the accuracy, at high temperatures and 

 low densities, of the laws deduced by Maxwell, Meyer, Lo- 

 schmidt, Stefan, and Boltzmann, which for ordinary tempera- 

 tures and densities hold good in gases; they find, first, that 

 the co-efficient of sliding friction between moving gas and a 

 fixed plane has a determinate value dependent on the nature 

 of the gas, so long as this is present in layers thicker than 

 fourteen times "the mean length of path of the molecules" 

 as defined by the kinetic theory of gases ; the co-efficient is 

 also inversely proportional to the pressure. Second, the ab- 

 solute value of the co-efficient of sliding friction is found 

 to be 0.7 x^, on the assumption that the molecule of gas 

 is reflected from the moving surface used in the apparatus 

 with velocities of translation equal to those of the surface 

 itself. For air at 760 millimeters, =0.000083 millimeter, 



H 



