New Scientist - 21.09.2019

54 | New Scientist | 21 September 2019

Lightning bulb

A summer storm woke me around

2 am. I heard a sizzling sound before

lightning struck about 100 metres

away. Then I saw a 1.5-volt solar-

powered outside light glowing like

a 50-watt bulb. It faded after a few

minutes. What caused the sizzling

sound and made the light glow

so brightly?

Storm Dunlop

East Wittering, West Sussex, UK

The sizzling sound was almost

certainly a corona discharge and

is often heard before a nearby

lightning flash. The discharge

occurs when the electrical

field between the cloud and the

ground is strong enough to cause

electrons to be emitted from the

tips of any pointed objects – even,

in some cases, from people’s hair.

As for why the light glowed, it

depends on the type of lamp. If it is

one that glows after being charged

during the day, the flash may have

provided sufficient charge for the

light to come on. If it is activated

by passive infrared (PIR) radiation,

it is possible that the heat from

the lightning was sufficient to

activate the PIR sensor.

John Woodgate

Rayleigh, Essex, UK

Both effects are due to the electric

field that is generated between

the thundercloud and the ground.

The light is caused by a current

that is induced in the wiring of

the lamp by the varying electric

field through its accompanying

magnetic field. These fields persist

for a while at a lower level than the

very strong fields created just

before and during a strike.

Anthony Richardson

Ironbridge, Shropshire, UK

It is unclear whether the bulb

was powered by the intense light

absorbed from the lightning by

the solar panel or, in a less likely

scenario, by charge absorbed

from the air. The sizzling sound

must be the sound of the air

being explosively expanded

in the lightning channel.

Hard-baked

Why do crisp ginger biscuits go soft

if left exposed to the air for a couple

of days when other baked products,

such as cakes and bread, go hard?

Krista Nelson

Rokeby, Tasmania, Australia

The difference between a cake

and a biscuit is similar to the

difference between bread and

toast. Bread starts out soft and

moist but dries out over time

and becomes an unpleasant mix

of soft and dry. Toast is made by

drying out bread to make it crisp.

As toast ages, it absorbs moisture,

making it rubbery and unpleasant.

Biscuits are the toast of the

cake world and were originally

made by baking cakes twice to

dry them out for storage. If you

leave a cake and a biscuit in a

standard kitchen, the moist

cake will dry out and the dry

biscuit will become soggy, both

approaching the same state, just

from different directions.

Claire Gregson

Portadown, County Armagh, UK

Biscuits are essentially dried

cakes, so absorb ambient

moisture. Cakes are much more

moist, so evaporate water to the

surrounding air. Just eat and enjoy.

David Jackson

Liverpool, UK

Biscuits start out with a very low

moisture content of between 1 and

3 per cent, depending on the type,

whereas this is around 15 to 30 per

cent for cakes. In an atmosphere

of moderate humidity, water

will diffuse out of a cake and

into a biscuit until equilibrium

is reached, not only with the

entrapped air, but also the starchy,

sugary matrix of the product.

In the 1990s, biscuit and cake

manufacturer McVitie’s fought a

UK tax claim on its Jaffa Cakes.

Chocolate-coated biscuits are

subject to value-added tax (VAT)

in the UK, whereas chocolate-

coated cakes aren’t, so a huge

amount of money was at stake.

McVitie’s (my employer at

the time) won the case, partly

because cakes, including Jaffa

cakes, become dry when they go

stale, whereas biscuits go soft.

Gerald Dorey

Oxford, UK

This question is an example

of the profound cake-biscuit

existential problem exemplified

by the Jaffa Cake. This chocolate-

coated confection has provoked

much discussion: a BBC radio

programme that discussed the

subject even invoked Ludwig

Wittgenstein’s ideas of the futility

of “family resemblance” tests,

whether a categorisation is merely

a semantic reflex and whether

a non-binary category might

be applied.

Lawyers are more practically

minded, and in 1991, a UK VAT

tribunal decided that the critical

factor was whether the item

absorbs moisture over time and

becomes softer, as with biscuits,

or gives up moisture and hardens,

as with cake. This confirmed that

the Jaffa Cake is a cake, and is

therefore liable for a lower rate

of tax than it would have been

if classified as a biscuit.

Clearly, the ambient humidity

is critical. I would suggest that in

countries with weather that is

hot and humid enough to melt

the chocolate on a Jaffa Cake, no

cake would harden, nor would

a biscuit stay hard for long. The

reverse would be true in deserts.

In both areas, the cake-biscuit

duality disappears.  ❚

This week’s new questions

Sitting pretty We are frequently told we need plenty of

exercise and that sitting is bad for us. Is the problem with

sitting merely that it stops you exercising, or is sitting bad in

itself? John Gordon, Datchworth Green, Hertfordshire, UK

Coil conversion If I compress a metal spring, tie it with an

acid-proof binding then submerge it in acid and dissolve the

spring, what happens to the energy that was used to compress

it? I think the acid must warm up, but how is the stored energy

converted to heat? Roger Key, Bedale, North Yorkshire, UK

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