8 Prof. J. C. PoggendorflP on the Induction Apparatus 



fers from the rotating current-breaker, where a straight electro- 

 magnet oscillates horizontally on a point between the upright 

 arms of a steel horseshoe magnet, and where the ends of the 

 encircling wire dip into two semicircular mercury cups, con- 

 nected with the poles of a galvanic battery, and separated by 

 an insulating partition. Here the pauses are of much shorter 

 duration than the moments of activity of the current, because 

 the former occur only when the ends of the wire pass over the 

 partition. The construction of this rotating current-breaker 

 might, indeed, be so changed as to make the pauses equal in 

 duration to the periods of activity ; but to do so would be to 

 render the instrument more expensive, and as this equality was 

 not necessary in the present investigation, I have continued to 

 use NeePs hammer. 



Condenser. 



At first my condenser was exactly similar to the one con- 

 structed by Ruhmkorff, probably according to Fizeau's own in- 

 structions ; it consisted of a long piece of oiled silk covered on 

 both sides with tinfoil, and folded to and fro in layers so as to 

 occupy less space, and to allow only parts of one and the same 

 sheet of tinfoil to touch each other. Each of these sheets of tin- 

 foil was 8 feet long and 11 inches broad. Soon afterwards a 

 second condenser, about one- fourth the size of the first, viz. 23 

 inches long and 11 inches broad, was used; the folded layers 

 were separated from one another by mill-board in order to pre- 

 vent contact between the posterior surfaces of the tinfoil, which, if 

 present, would necessarily be prejudicial. 



Afterwards I became acquainted with the simplification pro- 

 posed by Halske, wherein the voluminous arrangement in Ruhm- 

 korff's apparatus is replaced by a single covered sheet of mica 

 not greater than the octavo form. I have had an opportunity of 

 testing a condenser of this kind and of this magnitude prepared 

 by Prof. Riess, and I thereby convinced myself, that, under the 

 circumstances under which the test was applied, its effect was 

 very nearly equal to that of the great oiled silk condenser. 



This great superiority of the mica condenser, in compai'ison 

 to its magnitude, is caused by one of the sides of the condensing 

 surfaces being in free contact with the air, and by the two 

 surfaces being nearer to one another than was the case with 

 oiled silk, which, besides being somewhat thicker than the plate 

 of mica, is so bent and folded as to prevent the close proximity 

 of the sheets of tinfoil. 



Slica is an excellent substance for such condensers, but it 

 cannot readily be procured in plates of the requisite size. This 

 caused me to search for substances capable of replacing it. 



