PROPERTIES OE THE LEAF OF DIONAEA. 15 
other with one hole. On the vulcanite rail runs a brass sliding block or ridei, the 
form of which is seen in fig. 5. It rests on the surface ol the graduated wire by its 
cross wire, and is connected with the binding screw by a flexible wire. From the 
construction it is obvious that the difference of potential between the binding screws 
B and C, is to the total difference between A and B, as the length of wire, in millims. 
between B and the point at which the cross wire rests on the graduated wire is to 
5000 millims. In other words, that each millim. of the graduation corresponds to 
smooth of the total difference. In use, A and B are connected by one pair of wires 
with the standard cell through a galvanometer, by the other pair with an “ auxiliary 
battery ” through a rheostat as described in Mr. Latimer Clarke’s paper. 
Measurement of eleetrieal resistance. —It is of great importance to have the means of 
becoming promptly aware of the changes of resistance which occur in the course of a long 
observation. For this purpose we adopt the plan of from time to time measuring the 
deflection due to 10 millims. of wire = gtjo Daniell. In investigating plant struc¬ 
tures that method is pratically of great value, for the resistances we have to encounter 
vary, roughly speaking, between 50,000 and 100,000 ohms. As, therefore, we know 
approximately that our circuit, when the electrodes are in contact, is about 12,000 ohms, 
viz.: 6000 for the galvanometer and about as much for the electrodes, such changes as 
often occur are obvious enough. In the mode of leading off used in the fundamental 
experiment (see Part IV., p. 25) we obtain, if the contact is successfully made, with an 
over compensation of Daniell a deflection of about a tenth of the deflection due 
to the same electromotive force when the electrodes are in contact with each other. 
If it is less than that, we regard the leading off as unsatisfactory. For actual mea¬ 
surement, we have at hand on an adjoining table a Wheatstone’s bridge arrangement, 
which can be brought into connexion with our circuit without loss of time when 
required, by means of a switch which is substituted for the key which otherwise 
receives the wires from the leading off electrodes.* 
* This corresponds to K 3 in the diagram. Here L represents the leaf, / and m the leading off contacts, 
x and x' the exciting contacts, I and II the primary and secondary coils, R the rheotome, C the com- 
Diagram of Connexions of Compensator. 
