Coherence and Recoherence. 167 
described in the former paper. The suspended coil © of a 
D’Arsonyal galvanometer is suitably wound, and to its frame 
is soldered a boom A of copper wire. The boom projects a 
short distance and is perpendicular to the coil. A similar 
wire B is vertical and is long enough to make contact with 
A as shown. Coherence is produced at the single contact 
where A and B meet. M is a mirror by which a spot of 
light is made to indicate the position of the boom. ‘The sus- 
pensions of the coil are of thin strip phosphor-bronze, and 
the connexion between F' and A is a long loose piece of thin 
phosphor-bronze. D and EH are leads for the coil. The 
circuit is completed through a battery and resistances (a dial 
pattern rheostat is used to give slow gradational changes in 
resistance). F and G are the leads for the contact-circuit. 
In this cireuit we have a battery, resistances, galvanometer, 
and as shown a voltmeter V as shunt to the contact. The 
voltmeter, reading to +4, volt, is a convenient indicator. 
It shows whether there is (1) full coherence, (2) partial co- 
herence, (3) pressure between A and B without current 
passing, or (4) pressure between A and B with current 
passing. 
This apparatus is very sensitive, but has the disadvantage 
that it is difficult to clean or remove the contact attached to 
the coil without breaking the delicate strips of phosphor- 
bronze of the suspension. The following device, though less 
sensitive, has no delicate suspension and is more handy. 
Fig. 2. 
The Balance Apparatus (fig. 2).—Here DH is the variable 
contact. AB isa tube made of thin sheet aluminium. At 
B it is continued in a vertical helix F through which an 
ordinary sewing-needle Z is thrust. Into the end A of the 
tube a copper wire Dis thrust. Through the point E another 
