166 



DOVE OX THE ELECTRICITY OF INDUCTION. 



have a keeper specially coiled for each, it will be found advanta- 

 geous to use the second combination for such modes of measure- 

 ment as offer great resistance to conduction, and the first for 

 such as offer but little resistance. Now, as the human body 

 offers the greatest resistance of all the modes of measuring the 

 current that have been applied for the purposes of physiological 

 experiment, the successive connexion is to be preferred to the 

 parallel ; on the contrary, the parallel connexion will be found 

 more applicable for producing incandescence in platina wires and 

 charcoal points, for the sparks which accompany the interrup- 

 tion of a short connexion, and for the magnetization of sofl iron 

 which is enclosed in a connecting spiral. Which of the two com- 

 binations is preferable for chemical decomposition will depend, 

 for a given thickness of wire, upon the distance between the 

 electrodes of the voltameter, and upon the resistance to conduc- 

 tion offered by the electrolyte. The parallel combination of the 

 wire coils may therefore be called with as much right the physi- 

 cal, as the successive connexion is called the physiological. The 

 apparatus which effects both kinds of connexion by turning a 

 hand, and which may be called a pachytrope, is not represented 

 on the square piece of wood attached to the keeper in fig. ^, in 

 order not to complicate the drawing, but it is depicted by itself 

 in the following woodcut, and in the first position of the move- 

 able hand on the right side for continuous connexion, i. e. for 

 physiological action, in the second, for parallel connexion, i. e. for 

 physical action. 



The disc of copper, which is exactly divided into two halves, 

 is fixed upon a wooden support, and can be turned by means 

 of the handle under the four plates of copper, so that the 

 handle is alternately in contact with the left or the right upper 

 plate, a b are the ends of the one wire coil, «/3 those of the 

 other, jo is a connecting wire on the plate, under which b is 



