September 6, 1889.] 



SCIENCE. 



159 



AN IMPROVED STANDARD CLARK CELL WITH LOW 

 TEMPERATURE COEFFICIENT.! 



Lord Rayleigh's form of Clark cell, described in the Philo- 

 sophical Transactions for 1885, is the best one hitherto made. 

 The objections to it are, first, that it has a high and variable tem- 

 perature coefficient ; second, it is not constructed in such a way 

 as to keep the mercury away from the zinc when shaken in trans- 

 portation ; and third, an important chemical defect is the local ac- 

 tion taking place by which zinc replaces mercury in the mercury 

 salt and the zinc becomes amalgamated, the amalgam often creep- 

 ing up so as to reach the solder at the copper wire. These diffi- 

 culties I have, I think, perfectly overcome. I have made cells 

 which have been tested for several months with the low coeffi- 

 cient, at 15° C, of 0.000386 per degree C. At higher temperatures 

 a peculiarity is that this coefficient decreases slightly, while that of 

 Lord Rayleigh's increases very appreciably. The cell is so made 

 that the mercury is confined to the bottom of the cell, or at least, 

 if it does move at all it cannot reach the zinc. These cells have 

 been found to stand transportation exceedingly well. 



The same arrangement or device removes the zinc from the 

 mercury salt and perfectly prevents local action. The sealing of 

 the cell is also effected with a more perfect compound. Further, 



IMPROVED STANDARD CLARK CELL. 



in the preparation of the mercury salt I have succeeded in making 

 mercurous sulphate so free from the mercuric form that it shows 

 no yellowing when washed free from acid. It also remains white 

 upon admixtion with zinc sulphate, and indefinitely, after the cell 

 is set up, provided it be kept out of the light. The light dark- 

 ens it. 



One of these cells has been heated up to 53° C, and the follow- 

 ing day it returned to its precise former value of electromotive 

 force at the same temperature. The temperature coefficient given 

 holds at the above high temperature. As indicating the uniformity 

 attained, the last two cells made never differ in electromotive force 

 by more than one part in ten thousand, and usually by only half 

 this, at the same temperature. 



THE WENSTROM DYNAMO. 



The Wenstrom dynamo, of which Fig. i is a perspective view 

 and Fig. 2 a cross section, is well known in Europe, especially in 

 Sweden. It was invented by Jonas Wenstrom, an eminent Swed- 

 ish engineer, and differs in some respects from other dynamos in 

 the market. It is of simple and substantial construction, as may 

 be seen by the illustrations, and utilizes the magnetic forces to a 

 remarkable degree. It is of the iron-clad type, the armature and 

 field coils being protected by a cast iron shell, parts of which per- 



1 Abstract of a paper read before the American Association for the Advancement 

 of Science, at Toronto, by Professor H. S. Carhart. 



form the function of pole-pieces. There are four poles, opposite 

 ones being of the same sign, all four being energized by one pair of 

 field- coils, which surround the cores of the inner or horizontal pole- 

 pieces, and are surrounded by the shell which serves as annular 

 cores for the top and bottom field-pieces. Ventilation is provided 



FIG. I— WENSTROM DYNAMO. 



for by the circular apertures in the shell through which the arma- 

 ture is put into place. 



The armature is of the drum pattern, built up in the usual way 

 of thin disks of iron, well insulated, so as to prevent heating from 

 eddy currents. These disks are perforated near the periphery, the 

 perforations being round, ovoid, or hexagonal in shape, and con- 

 nected with the periphery by a slit, narrowest at the outer part, 

 and only wide enough to admit the winding, one wire at a time. 

 In the grooves formed by these perforations the wire is wound. 

 This peculiar construction admits of the armature revolving in very 

 close proximity to the pole-pieces, materially reducing the resist- 

 ance of the magnetic circuit, and affording a protection to the 

 armature winding from the effects of centrifugal force, no binding 

 wires being required. A new method of winding is employed, and 

 diametrically opposite sections are connected together, making nec- 

 essary only two brushes, which are set 90 degrees apart. 



The one hundred light machine absorbs eight horse-power, run- 



FIG. 2.- WENSTROM DYNAMO. 



ning at a speed of nine hundred revolutions per minute. The 

 total weight of the dynamo is eleven hundred pounds, mainly cast- 

 iron, the weight of copper wire on the armature being only thirteen 

 pounds, and on the field magnet cores ninety-four pounds, or one 

 hundred and seven pounds of copper in all. The two hundred 

 and thirty light machine runs at a speed of seven hundred revolu- 

 tions per minute, its total weight being twenty -five hundred 

 pounds, of which thirty-six pounds are of copper on the armature, 

 and three hundred and eight on the field magnets. The eight 

 hundred light machine runs at a speed of five hundred revolutions 

 per minute. 



The advantages claimed for this construction are that there is 

 no waste field, all the magnetic lines of force being utilized in the 

 armature in producing work ; neither is there any field outside of 



