LECT. XII. XIII. BREGUET'S APPARATUS. 239 



limb returns to its natural condition. This contraction, 

 which occurs when we close the circuit of the inverse cur- 

 rent, does not exist longer than the first moments of the 

 experiment, and it has re-appeared after the very prolonged 

 action of the same current. 



It was desirable to ascertain whether the contractions ob- 

 tained on opening the circuit of the inverse current, in- 

 creased within certain limits, in proportion to the time the 

 current remained closed. To ascertain this it was neces- 

 sary to measure the contraction, and this I did by means of 

 an apparatus contrived by Breguet. 



This apparatus consists of a solid brass support, A B, 

 fixed upon a wooden stand in which slide two pieces of 

 metal, c D, capable of being fixed in different places by 

 means of screws of pressure (vis depression.) The piece of 

 metal, c, is furnished with a vice, E, in which is to be held 

 the morsel of spinal marrow of the prepared frog, and is 

 fastened there by three screws. The other piece, F, of a 

 fork-shape, is provided with a hole in each extremity of the 

 fork, in which a very fine wire, G, is fixed or regulated. 

 At one end of the wire is a fork, to which is affixed the 

 claw of a frog ; the other end of the wire is attached to a 

 silken thread, which winds round the little pulley, i. Upon 

 this another thread of silk is wound in contrary sense to the 

 former, and to this attached a small leaden weight, o. The 

 axis of the pulley is furnished with a kind of double index, 

 p Q, in the form of a semicircle. The axis is fixed upon 

 two pivots, which admit of being more or less approxi- 

 mated. One of these pivots is the centre of a circle, R s, 

 which bears a division. A long ivory needle, T v, is at- 

 tached to this pivot ; it is very light, and turns with the 

 slightest possible touch. The use of this ivory index is ob- 

 vious. In effect when this index is brought in contact with 

 the semicircular one, p Q,, which is attached to the axis of 



