of equal intensity. (3) And for certain further experiments it
will be necessary to have some way of gradually increasing or decreasing
the stimuli in a definite manner.

#Modification of the block method.#--By consideration of the following
experimental modifications of the block method (fig. 55), it will be
found easy to construct a perfected form of apparatus, in which all
these conditions are fully met. The essentials to be kept in mind were
the introduction of a complete block midway in the wire, so that the
disturbance of one half should be prevented from reaching the other, and
the making of a perfect electrolytic contact for the electrodes leading
to the galvanometer.

Starting from the simple arrangement previously described where a
straight wire is clamped in the middle (fig. 55, _a_), we next arrive at
(_b_). Here the wire A B is placed in a U tube and clamped in the
middle by a tightly fitting cork. Melted paraffin wax is poured to a
certain depth in the bend of the tube. The two limbs of the tube are
now filled with water, till the ends A and B are completely immersed.
Connection is made with the non-polarisable electrodes by the side
tubes. Vibration may be imparted to either A or B by means of ebonite
clip holders seen at the upper ends A B of the wire.

[Illustration: FIG. 55.--SUCCESSIVE MODIFICATIONS OF THE BLOCK METHOD
FROM THE 'STRAIGHT WIRE' (_a_) TO 'CELL FORM' (_e_)
When A is excited, current of response _in the wire_ is from less
excited B to more excited A. Note that though the current of
response is constant in direction, the galvanometer deflection in
(_d_) will be opposite to that in (_b_).]

It will be seen that the two limbs of the tube filled with water serve
the purpose of the strip of moistened cloth used in the last experiment
to make electric connections with the leading-out electrodes--with the
advantage that we have here no chance of any shifting of contact or
variation of surface, the contact between the wire and the surrounding
liquid being perfect and invariable.

On now vibrating the end A of the tin wire by means of the ebonite clip
holder, a current will be found to flow from B to A through the
wire--that is to say, towards the excited--and from A to B in the
galvanometer.

The next modification (_c_) is to transfer the galvanometer from the
electrolytic to the metallic part of the circuit, that is to say, it is
interposed in a gap made by cutting the wi