PHYSICS. 377 



that whatever be the uumber of surfaces of zinc in metallic communi- 

 cation in any 0ven element, and whatever be their distances from the 

 inactive electrodes each of these surfaces contributes to the general 

 useful action very appropriately in the inverse ratio of the square of 

 its distance from this electrode. (II Nuovo Cimento, xviii, 138; J. 

 Phys., December, 1886, II, v, 573.) 



Hopkinson has presented a paper to the Royal Society on dynamo- 

 machines, the purpose of the investigation beiug to give an approxi- 

 mately complete construction of the characteristic curve of a dynamo 

 of a given form from the ordinary laws of electro-magnetism and the 

 known properties of iron. Taking the curve already determined for 

 wrought-iron and constructing a characteristic in this way, he has ob- 

 tained a theoretical curve which agrees over a long range with the 

 actual results of observations on a dynamo-machine more closely than 

 any empirical formula yet published. (Nature, May, 1886, xxxiv, 20). 



Gore has examined experimentally the effect of temperature upon the 

 Peltier effect, and finds that with couples formed of bismuth-antimouy, 

 iron, German silver, and bismuth-silver, the total Peltier effect is greater 

 in each case at the higher temperature than at the lower one, but with 

 the antimony-silver couple the effects at the two temperatures were 

 about equal. (Phil. Mag., April, 1886, V, xvi, 359.) 



3. Electrical units and measurements. 



Lorenz has experimented to ascertain whether the resistances of 

 liquid columns of the same length vary exactly in the inverse ratio of 

 their sections when these sections are quite small. By measuring di- 

 rectly the resistances of non-capillary mercury columns and comparing 

 them with those of capillary columns contained in tubes calibrated with 

 the greatest care, he finds a difference in the latter of from 14 to 21 per 

 cent, less than the calculated values. This difference, if real, is too 

 small, however, to affect the absolute resistance except in the thou- 

 sandth's place. The author's experiments fix the absolute value of the 

 ohm at 105.93 centimeters ; differing only by one-thousandth part from 

 the conference unit. (Wied. Ann., xxv, 1 ; J. Phys., November, 188C, 

 II, V, 539.) 



Eayleigh has suggested certain criticisms on the details of the meth- 

 ods emi^loyed by Hinstedt in the determination of the ohm. He him- 

 self has found mercury contacts to be unreliable, and has substituted 

 platinum contacts for them. Moreover, the question should be consid- 

 ered whether the axial magnetization of the needle does not alter under 

 the action of a force having a sensible axial component. And, again, 

 the methods of winding and measuring the primary and secondary coils 

 introduce sources of error in his opinion. (Phil. Mag., January, 1886, 

 V, XXI, 10.) 



In a paper on the Clark cell as a standard of E. M. F,, Rayleigh has 

 discussed the relative advantages of various modes of preparation. The 



