156 THE SIGNS OF LIFE 



The simplest way of obtaining this arrangement is to have a 

 keyboard with several pairs of terminals, to which are connected 

 the various parts of apparatus, making up what has already been 

 called the normal circuit. Each particular piece of apparatus is 

 controlled by a plug, opening or closing the interval between the 

 two terminals to which each pair of wires is connected. Two 

 commutators, one in the exciting circuit and the other in the 

 compensator circuit, allow the current to be sent in the direction 

 desired. An ordinary key interrupts the principal circuit of the 

 compensator. 



Note. It much simplifies matters to arrange the circuit 

 permanently, or at least at the beginning of each experiment, so 

 that a positive or negative current may be in conventional 

 directions, i.e., positive to the right and negative to the left. 

 The quickest way of determining the direction of a current is 

 to touch ,one of the terminals with a piece of metal (e.g., zinc) 

 held in one hand, while a finger of the other hand makes contact 

 with the other terminal. The terminal touched by the zinc " pulls " 

 through the galvanometer, and if the previous deflection has been 

 in the same or in the opposite direction, we know that the spot 

 in connection with the same terminal was then zincative or counter- 

 zincative, i.e., electro-positive or electro-negative. 



Photogaphic recording. If necessary, it is possible to photograph 

 and to take readings at one and the same time. To this end the 

 transparent scale must be replaced by a vertical opaque screen, 

 with a narrow horizontal slit, behind which a photographic plate 

 is let down by clockwork. 



The use of two galvanometers in series so simplifies matters, 

 however, that it is preferable to use a second instrument for taking 

 graphic records. One of the galvanometers stands in the laboratory, 

 with its transparent scale in front of the observer, while the second 

 is some distance away in a dark (and non-vibrating) room. The 

 former exhibits and the latter registers the currents under obser- 

 vation. 



The graduation of the recording galvanometer should be to a 

 smaller scale than that of the indicating galvanometer. A con- 

 venient relation between the two scales is I to 10, so that each 

 centimetre deflection of the indicator is represented by a millimetre 

 on the recorder, The relation is, if necessary, adjusted by shunting 



