C. GENERAL PHYSICS. 151 



according to the amount of light passing through the sub- 

 stances. When a strong light from the ignition of magne- 

 sium was made about nine inches above the surface of a basin 

 of water, at the bottom of which a selenium bar was placed, 

 the resistance of the latter immediately fell more than 70 per 

 cent., but returned so soon as the light w^as extinguished. 

 4 1>, 1873, 301. 



EFFICIEXCY OF DIFFERENT ELECTRICAL MACHINES. 



Mascart has presented the results of some comparisons 

 made by him as to the efficiency of the different styles of elec- 

 trical machines, in comparison with the work done in turn- 

 ing them. The efficiency of a machine is defined by, 1, the 

 difference of electric potential that it is capable of establish- 

 ing between two conductors; 2, the quantity of electricity it 

 can deliver in a given time. Por the Ramsden plate ma- 

 chine the efficiency is proportional to the diameter of tlie 

 phate : the cylinder machine of Xairne is but one half so 

 powerful. The Holz induction machine, having two mov- 

 able plates, rotating in the same direction, exceeds all the 

 friction machines. The electricity furnished by the Rulim- 

 korff induction coil of large size, however, exceeds the Holz 

 machine in the ratio of one hundred and thirty to eighty-six. 

 One spark from the inductive coil, the electrodes being twenty 

 centimeters apart, contains about the same quantity of elec- 

 tricity as is given by one turn of the handle of the Holz ma- 

 chine ; at the usual rate of operation of the Ruhmkorff appa- 

 ratus, which is six sparks per second for strong discharges, 

 it becomes equal to tliree Holz machines. 6 -^, 1873, 1014. 



THE EFFECT OF GALVANIC CURRENTS ON THE LENGTH AND 

 ELASTICITY OF A METALLIC CONDUCTOR. 



It is known that any Avire w^hich conducts a galvanic cur- 

 rent is somewhat heated up thereby, and in consequence 

 changes its elasticity and its length. Streintz has undertaken 

 to investigate whether a wire that is heated up to the same 

 temperature by external means, and without conducting elec- 

 tricity, experiences the same change in its length and elas- 

 ticity as do those wires through which a galvanic current is 

 passed. Previous investigators, especially Wertheim and 

 Edlund, have with reference to this question arrived at op- 



