OF ARTS AND SCIENCES. 



43 



that shown in Figure 4, in which 5 is a block of hardwood about 

 14 cm. long on the upper curved edge, 6 cm. thick, and 5 cm. wide. 

 The brass spring S fastened to the top of the block is connected by- 

 means of the wire I with the galvanometer. The brass spring S' 

 also fastened to the top of the block, is connected by means of the 

 wire I' with one of the junctions i or ?i in the pot of melted paraffin. 

 The two springs do not at present touch each other, but will do so 

 at P, as soon as S' is lifted by the lever LL, which is pivoted at 

 a, and will be struck at E by the cam K on the engine crank. 

 The lever LL works in a narrow slot sawed in the block B. To 

 prevent short circuiting through the body of the engine this lever 



>^b.^ 



is tipped with hard rubber where it touches the spring /S'. The 

 electromotive force which one has to deal with in this experi- 

 ment is so small that a mere touch of S' against S is not sufficient. 

 Contact must be maintained for a short time, and for this purpose 

 the lever LL is so shaped that the cam K rubs against it all the 

 way^ from LJ to D, about one sixtieth of a revolution, one sixtieth 

 of a second in my experiments. To prevent illegitimate contacts 

 or illegitimate breaks between /S and S', each of these strips is 

 controlled by two stiff spiral springs, which are indicated by Cfor 

 S, and by C for S'. The motion of S is, moreover, limited by 

 stops, not shown in the figure, placed above and below at the end. 

 The lever LL is made of thin sheet brass in order that its mo- 

 mentum may not be troublesome, but the part LJD, along which 

 contact with K endures, is reinforced by a narrow strip of iron 

 soldered on. A pin, ^j, keeps LL in contact with S' when both 

 are at rest, and prevents the lever from dropj^ing so far as to be 

 struck too hard by K. 



