PHYSICS. 121 



Du Moncel lias studied the relation which should exist 

 between the diameter of iron cores and the thickness of their 

 magnetizing helix, and finds from experiment that there is 

 an advantage in winding electro-magnets so that the thick- 

 ness of the coil-layers is equal to the diameter of the cores. 

 Moreover, the diameter of the cores should naturally be pro- 

 portioned to the electric intensity which is to act on them, 

 and be so chosen that they shall be nearly saturated by the 

 current. 



Helmholtz has communicated to the Academy of Berlin a 

 paper containing the results of experiments by Rowland, 

 which satisfactorily prove that electric convection currents 

 are dynamically equivalent to the flow of electricity in a 

 conductor, and are electro-magnetically operative. 



2. Electromotors. 



Zollner has investigated a new class of electrical phe- 

 nomena hitherto but imperfectly known. When two dif- 

 ferent bodies, an insulator and a half- conducting: rubbing 

 instrument, are rubbed together, electrical currents occur in 

 the rubber as follows : if the rubbed insulator be positively 

 electric, the currents at the surface of contact or in the inte- 

 rior of the rubber are parallel, but opposite to the relative 

 motion of the insulator; if the latter be negative, the cur- 

 rents of the rubber are parallel and in the same direction as 

 the insulator's motion. These currents were measured, and 

 shown to be often very considerable. They could be inten- 

 sified by multiplying the rubbers and connecting their cor- 

 responding parts with wires. They lessen the useful effect 

 of an electrical machine, in which, however, a positive ad- 

 vantage is had by uniting the electricity of the positive end 

 of the rubber with the positive electricity of the conductor. 

 The author has also studied a variety of related experi- 

 ments, as, for example, the currents generated by the flow 

 of water through a thin tube. Zollner concludes that dia- 

 phragm currents and their modifications are due to the oc- 

 currence of new electro-motive forces, such that the electric 

 current they generate in the moved liquid, so long as it is in 

 contact with the canals of the diaphragm or the capillary 

 tube, are always opposite to an electric current which would 

 force the liquid in the same direction through the diaphragm 



F 



