TRANSACTIONS OF SECTION G. 569 
WEDNESDAY, SEPTEMBER 11. 
The following Papers were read :— 
1. The Experimental Determination of the Stresses in Springs and other 
Bodies by Optical and Electrical Methods.‘ By Professor E. G. 
Coker, M.A. 
Usually the chief difficulties in determining the state of stress in a body arise 
from the great variations of stress intensity which occur owing to its complicated 
shape and the loading; and, further, in the case of springs where plates are built 
up into a matrix, the rubbing friction between the surfaces is usually consider- 
able enough to render the assumptions of perfect elasticity of the whole body 
somewhat unreliable for purposes of calculation, although each plate may be 
regarded as fulfilling the elastic conditions perfectly. 
Methods of experiment were described by optical and electrical methods of 
general application to stress problems. 
In the first method, models of springs are constructed of transparent materials 
for which it is shown that the stress distribution is very similar to that in 
steel. These models permit of determinations of the difference of principal 
stresses at any point, and an important result is that the optical effect of a pure 
shear is proportional to twice the numerical value of one of the principal stresses. 
Examples of plate springs and flat-coiled springs were considered in detail, 
and the general distribution of stress illustrated by diagrams and natural- 
colour photographs of springs viewed in polarised light. 
The second method depends on the fact that steel and other metals when 
subjected to stress within the elastic limit experience a change of temperature— 
a diminution for tension stress and an increase for compression stress—propor- 
tional to the stress. The effect at any point, therefore, is due to the sum of the 
principal stresses, and in cases of pure shear the effect is zero—a noteworthy 
difference from the first method. Examples of tension, compression, bending, 
em experiments were described showing the applications of the electrical 
method. 
The paper described some attempts to utilise the difference in the electrical 
nee of stressed and unstressed metals for determining the stresses. in 
materials. 
2. Alternating Load Testis.” By Brrnarp P. Haran, B.Sc. 
This paper dealt with the testing of wire specimens under pulsating loads, 
the pull being applied in a sine-wave by a machine comprising an electro-magnet 
supplied with alternating current. The pull of such a magnet varies between 
zero and a maximum in a sine-wave with twice the frequency of the alternating 
current supplied, and the maximum pull is proportional to (E + C)?*, where E 
and C are respectively the voltage and frequency of the current. 
The wire specimen is attached at its lower end to an armature which vibrates 
above the pole-face of the magnet, the magnetic circuit being arranged so as to 
give as light an armature as practicable. The conditions necessary to ensure 
that the pull is independent of the range of vibration are discussed, and also 
the means by which the forces absorbed in accelerating the mass of the armature 
in its harmonic motion are compensated. The vibrating armature is carried on 
springs adjusted to such a stiffness that the force exerted under any given 
deflection is equal to that required for the acceleration of the mass of the arma- 
ture in that amplitude at the particular frequency of the test required. In the 
instrument exhibited the springs are arranged to compensate at frequencies from 
35 up to 120 extensions per second, and are adjusted by an experimental method 
employing ‘resonance.’ The springs are also arranged to hold the armature in 
position, and as no guides are employed it is unnecessary to make any allowance 
1 See Engineering, September 20, 1912, 2 Ibid. November 22, 1912. 
