杏吧原创

Powerful sound

Question: Walking through the countryside last weekend I noticed the overhead
power lines were emitting a very loud hum. What causes this noise in
high-voltage cables?

Answer: The hum is the result of the alternating electric field affecting
water on the surface of the power line.

Possible mechanisms include vibration of the water surface caused by the
field, spitting from the tips of water droplets (corona discharge), or the rapid
expansion of the air at the tips, due to heating.

While the details of how the noise is actually produced are not clear, we do
know a good deal about what affects the level of noise. 杏吧原创s have measured
the spectrum of the noise, the effect of rain and the age of the power line, as
well as the voltage and the design of the line.

When an aluminium line conductor is new, it is shiny and slightly greasy.
Water beads up on it due to surface tension, but the shape of a water drop is
also affected by the local electrical field, which can be fairly close to the
ionisation point of air鈥攁bout 3 megavolts per metre. The water drops have
been seen to assume pointed, conical shapes because of the field, and their
sharp points strengthen the field locally, causing the breakdown of air
molecules at the tips.

With a new conductor and a strong field at the surface, the amount of rain
has very little effect on the noise. However, if the conductor is several years
old, its surface will be pitted and dull. Instead of beading on the surface, the
water penetrates the bundle of wires. So rather than having water beads all over
the surface, the conductor looks smooth. At a low surface field strength and in
very light rain, the noise is much quieter than for a new conductor in the same
conditions.

However, the noise level is very variable, and depends much more on the
rainfall rate. The heavier the rain, the louder the noise, which doesn鈥檛 stop
getting louder until the rain is very intense. Its spectrum includes higher
frequencies, so it sounds more like frying.

But water cannot accumulate indefinitely inside a conductor. Wires seem to
absorb water up to a critical level, and then release it suddenly. I have
actually witnessed jets that were more than a metre long emerging from the
bottom of an aged conductor, though this process is apparently silent.

Harold Kirkham

Sunland, California

Distant voices

Question: I recently saw a 鈥渟peaking tube鈥� at a science museum and read that
before the telephone was introduced, speaking tubes were often used to connect
the employees on different floors of large offices. Is this true? If so, were
they able to 鈥渟witch鈥� calls from one outlet to another around the building or
did all the calls just connect pairs of fixed points?

Answer: Speaking tubes did not have multiple connections. The tubing was made
from lightweight compo, or lead, pipe about 1.5 centimetres in diameter, which
would be fixed from point A to point B. At both ends there was a mouthpiece
fixed to the wall at head height, and each mouthpiece had a removable whistle
inserted into it, secured to the mouthpiece by a brass chain. The whole
apparatus was then attached to the wall at head height.

A caller at point A would remove the whistle, place their mouth against the
mouthpiece of the tube and blow hard. This sounded the whistle at point B to
attract attention. The whistle at point B would then be removed and very good
voice contact could be made through the tube.

Speaking tubes were still in use in the 1930s. In my own experience at an
ironmongers, my office had a tube that was linked to the shop counter, and a
totally separate pipe connected us with the goods outward department.

Speaking tube parts cost 2 shillings and 3 old pence a set (11.25 pence in
decimal currency), and the lead pipe was sold in 50-foot coils at 4 old pence a
pound. Early mouthpieces and whistles were made of turned hardwood, but later
models were made of Bakelite.

Ken Truswell

Picton, New South Wales

Answer: Aircraft carriers (on which I served as a pilot between 1941 and
1962) were honeycombed with numerous speaking tubes.

There was an all-important tube from the bridge to the wheelhouse, and there
were also tubes from the bridge to the radar control room, lookouts and
signalmen. To ensure immediate response there were tubes from the operations
room to the aircrew ready-room, and from flying control to the freight deck, to
name but a few.

They worked well and鈥攃rucially鈥攚ere completely independent of
electronics.

David Newbery

Oxted, Surrey

Answer: Speaking tubes were used in large buildings and also on naval ships.
They were only direct and could not be switched to other locations.

Because of this, some large battleships had a row of 鈥渉orns鈥� on the bridge,
linked to different parts of the ship. The ends of each tube were fitted with a
whistle, and the caller would first blow down the tube to alert the receiver.
This gave rise to the phrase 鈥渙n the blower鈥�, which now means on the
telephone.

Robin Johnson

Chatham, Kent

This week鈥檚 questions

Leading the sighted: Have there been, or are there currently, any successful
blind scientists? If so, what kind of research do they do?

Cindee Bulthaupt

Nutley, New Jersey

Thirsty work: Alcohol causes dehydration. How does it do this and how much
water do you have to drink to compensate for the effect?

When drinking a gin and tonic, does the water in the tonic compensate for the
effect of the alcohol, or does the fact that it is mixed with gin make it act
differently in some way?

Mark Hunter

Bristol