724 REPORT—1896. 
the British Association at its Southport meeting in 1883. Models showing right- 
handed and left-handed specimens of these crystalline molecules and the configura- 
tion in which they must be placed to form a rock crystal ending in its well-known 
six-sided pyramid are before the meeting to illustrate the present communication. 
In a communication which I hope to make to the Royal Society of Edinburgh 
at an early meeting essential details of the configurations now suggested, and of 
the mutual forces between the atoms required by the conditions to be fulfilled, will 
be considered, 
3. A Magnetic Detector of Electrical Waves. 
By E. RutHEerRForD, JA. 
It has long been known that a steel needle placed in a spiral round which an 
ordinary Leyden jar discharge is passed is magnetised. The magnetism of the 
needle is generally confined to the surface, and the way in which the magnetisation 
varies from the surface inwards may be directly determined by dissolving the 
needle slowly in acid before a magnetometer. 
If a magnetised piece of steel wire be subjected to the discharge, the magnetic 
moment is always reduced, whatever the direction of the discharge. The screening 
action of thin cylinders of metal for the discharge may be immediately shown by 
placing tnem between the solenoid and detector needle. With a thin copper 
P leria the needle remained unafiected, while a few turns of tinfoil gave a small 
effect. 
A short steel wire magnetised to saturation also has the remarkable property of 
being able to distinguish between the two first half oscillations of the discharge. If 
the needle is saturated, in one direction the first half oscillation can produce no effect 
on the magnetism of the needle, since it is already saturated, while in the opposite 
direction it produces its full effect. From the comparisons of the fall of magnetic 
moment of the needle in the two cases, the damping of the discharge may be 
deduced. By an application of this method also the apparent resistance of air 
breaks of different lengths to the discharge was deduced, and the resistance of iron 
wires for currents of high frequency of alternation obtained. Instead of a single 
wire a compound needle of short thin steel wires insulated from each other by 
paraffin was used. This was a sensitive means of detecting and comparing oscil- 
lation of small intensity. Ifa circle of wire 30 cm. in diameter be taken, and the 
discharge passed round only a small portion of its arc, there is quite a large effect 
on the detector needle at the centre. 
If a discharge is sent /ongitudinally through a short magnetised steel wire, the 
magnetic moment is always reduced, due to the circular magnetisation of the 
surface layers of the wire. Using a thin wire in series with the circuit, oscillations 
of very small frequency may thus be detected. 
A compound detector needle of fine wire placed in a solenoid of two or three 
turns is a very simple and conyenient means of investigating waves along wires 
and determining nodes and antinodes. 
A compound detector needle was also found to be a sensitive means of detecting 
Hertzian waves in free space at large distances from the vibrator. 
A collection of twenty or thirty fine steel wires, each about 1 cm. long, 
was taken and formed into a compound detector needle, each wire being insulated 
from the other to prevent eddy currents. A fine wire solenoid of several hundred 
turns was wound over it. When the small solenoid was placed in series with 
receiving wires, a wave falling on the receiver set up oscillations in that circuit, 
and the needle is more or less demagnetised according to the intensity of the wave. 
Using large vibrators effects were obtained at a distance of over half a mile between 
the vibrator and receiver. 
