168 Dr. Shaw and Mr. Garrett on 
fine needle is pushed, the two ends of the needle resting on 
two curved supports of the wire K. 
By pushing D in or. out of the tube, the beam can be 
balanced about E, and by pushing the needle up or down in 
F the beam can be rendered just stable. The beam is ob- 
viously very light, so that in AB we have all the conditions 
required in a sensitive balance, and the wire D can be made 
to rest with any small pressure on the wire H which crosses 
it at right angles. A coil GGis placed below the magnetized 
needle, so that by passing a suitable current in GG the 
needle is sucked into GG and the contact DH is broken. 
It is easy to see how by varying the current in GG the 
pressure between D and H can be changed at will. 
The voltmeter V is shown shunting the contacts as before. 
Whether we use Apparatus I. or II., it must be placed on a 
table supported by long rubber springs, to avoid extraneous 
vibration. This is indispensable, since even a small shock 
breaks the coherence, 
Hach apparatus has to be roughly calibrated for force, so 
that knowing what current is put on in the actuating coil 
GG (fig. 2) we may convert it into a known thrust or tension 
felt at the contact, according as the current is direct or re- 
verse. In the former paper the method of calibrating the 
boom apparatus was described. For the balance apparatus 
calibration is easy: we put a rider of known weight on the 
balance itself at such a place that its effect is equal to that of 
the current used in GG. 
Coherence. 
Possible explanations of the force with which two surfaces 
cling when cohered are :— 
1. The fusion theory (Lodge).—The alternating discharge 
from one surface to the other develops heat enough to fuse 
the solid surfaces, so that after the discharge a strong solid 
bridge remains. 
2. Branly’s theory.—The medium between the contacts 
undergoes a modification under the electromagnetic waves, 
which causes it to conduct more or less well. 
3. The adhesion theory (Auerbach).—Mechanical shock 
accompanying the discharge causes the particles to come 
into and remain in more intimate contact. 
4. The wonic theory.—Hither a film condensed from the 
atmospheric gases or else an oxide film is ionized by the 
passage of the discharge ; so that positive and negative charges 
will exist in or on the opposed films causing them to attract 
