1921 .] Marsden and Ferguson.—High-tension Insulators. 109 
Thus such a drop will tend to grow, and it follows that charged ions will 
become the centres of condensation of moisture.* 
It will be seen, therefore, that the charged ions due to corona from a 
high-tension circuit will cause moisture in the neighbourhood to condense 
on them rather than on the insulator itself. The effect of moisture on an 
insulator, as will be seen from the paper referred to above, is to seriously 
increase the risk of breakdown owing to absorption of this moisture by the 
porcelain either by capillary action or by electro-osmosis. The corona losses 
near an insulator of the type used in the Lake Coleridge system is specially 
marked in the corners near the cement underneath and between the 
different shells of the insulator. (Bee illustration, p. 178, fig. 1, of paper 
referred to.) This is no doubt due to the fact that these corners are near 
to a direct line joining the charged wire in the neck groove to the earthed 
pin. The electrostatic force in the air will be greater than that in the 
porcelain (along the same line of force) by a factor equal to the specific 
inductive capacity of porcelain— i.e., about 5. Thus there will be considerable 
potential gradients in the air near the cement corners under the shells of 
the insulators, with consequent corona effects and large ionization. This 
will tend to prevent the deposition of moisture at these junctions, which, 
because of their unglazed condition, are the most vulnerable points of the 
insulator. 
Another point of interest arises from this consideration. The velocity 
of an ion under a potential gradient of 1 volt per centimetre is about 
1*6 cm. per second and proportional to the potential gradient. Thus the ionic 
velocities in the corona will be very large indeed, and the motions of the 
ions communicated to the neutral molecules of air will cause local wind- 
currents. This will be pronounced in the corners referred to above. 
The following observations w6re made while experimenting on insulators 
for the purpose of ascertaining the effect of a coating of dry dust on the 
insulator-surfaces. A 66,000-volt insulator (this Journal , vol. 3, p. 178, 
fig. 1, 1920) was coated all over with fine road-dust, and pressure was 
gradually applied between the pin and a wire attached to the top of the 
insulator. As voltage increased and approached 40,000 volts it was 
observed that a movement of dust-particles was taking place near the cement 
joints, and on examination after the test it was found that while the 
insulator generally was dusty there was a perfectly clean zone extending 
outwards from the cement joints for a distance of about 1 in. The 
wind-action described above no doubt explains why the cement and 
insulator-surfaces immediately outside the cement joints, on this type of 
insulator, remain comparatively clean when in service on a transmission¬ 
line, while more remote surfaces become dirty. Mist would probably be 
repelled also and prevented from settling on the vital parts of the 
insulator. This would explain curves 2 and 3 on p. 180 of the article 
referred to above. 
We have seen that there will be a high degree of ionization underneath 
the shells of a charged insulator, and this will undoubtedly affect the 
potential of arc-over.. This consideration therefore suggests an improve¬ 
ment on the design of insulators by having a larger area of contact of the 
shells, and removing the junction to points more remote from the pin and 
neck-groove. 
* For a complete discussion of these effects see J. S. Townsend, Electricity in 
Gases , Clarendon Press, 1915. 
