1918.] The N.Z. Journal of Science and Technology. 
221 
hard wood block and sharp edge of the elbow : result, insulation is cut, and 
in due time a leak sets up, frequently causing an incipient fire at the back 
of the wood block. This trouble is liable to occur at any outlet-point where 
the metal box or bush has been omitted. 
Conduit runs must be electrically and mechanically continuous through¬ 
out their length, and efficiently earthed. “ Earthed ” means that the 
conduit shall be electrically connected with the general mass of earth in 
such a manner as will ensure at all times an immediate discharge of 
electrical energy without danger. This is done by connecting one end of a 
piece of copper wire to the conduit, and the other end to a water-pipe 
forming part of a town supply. Where there is no public water-supply 
a 6 ft. length of 1 in. iron pipe is driven into the ground, and the copper 
wire attached to that. Proper earthing is of the utmost importance not 
only to guard against the fire hazard, but also to guard against the danger 
of shock. If there is a leak on to the conduit and the conduit be not 
earthed, or only partially earthed, there is a grave risk of a very severe 
shock being received by any person touching it, especially if the floor is 
damp, as in a bathroom, or consists of earth or concrete. Again, if the 
person touched the conduit with one hand, and a gas or water pipe, or any 
metal connected with the earth, with the other hand or any part of the 
body, a severe shock would be the result, even if the person were insulated 
by a dry floor, carpet, or wooden box. The fire hazard in this case might 
be due to a partial contact with metal, or moisture round the conduit due 
to leaky roofs or washing of floors. 
Several types of grip-joint fittings have been introduced as competitors 
to the screwed conduit, but they are not so reliable, and, unlike the screwed, 
there is a limit to the situations in which they may be used. One of the 
principal objections is the liability of broken electrical continuity, resulting 
in bad earthing. Plain slip-joint conduit—that is, conduit having no grip 
or means of clamping .at the joints—is not permitted under any circum¬ 
stances. 
The next method usually employed for protecting conductors is by 
enclosing them in wood casing. It was used to a very considerable extent 
before being displaced by conduit, and in the days when the voltage seldom 
exceeded 100 or 110, but has come to life again owing to the high price 
of conduit brought about by the present war. I must say that I have 
no great favour for wood casing with the usual pressure of 230 volts, and 
many supply engineers prohibit its use on their systems. Casing should 
be made from hard woods, and conductors of opposite polarity must be 
kept apart by a continuous fillet. No joints or crossing of wires are 
permitted. It must not be buried in plaster, run under floors or behind 
wainscotting, and under no circumstances must it be used in damp 
situations. Moisture in wood casing is extremely hazardous, and wood 
casing should always be well painted, preferably with a damp-proof paint, 
both before and after erection. 
Other systems of wiring have recently been introduced, where the con¬ 
ductors are enclosed in a close-fitting sheathing of metal, and can be used 
satisfactorily when run on the surface and exposed to full view. 
Open wiring—that is, unenclosed conductors supported on porcelain 
insulators — should only be used in very damp situations, such as freezing- 
chambers, and only then when not liable to mechanical injury. The main 
causes of trouble arising out of open wiring are mechanical injury, 
