Why a sausage can do what your gloves cannot - Charles Wallace and Sajan Saini
In 2010, South Korea experienced
a particularly cold winter.
People couldn't activate their smartphones
while wearing gloves,
so they began wielding snack sausages—
causing one company to see
a 40% rise in sausage sales.
So, what could sausages do
that gloves couldn’t?
In other words,
how do touchscreens actually work?
In 1965, the first ever touchscreen
was invented
to help British air traffic controllers
efficiently update flight plans.
However, the technology was too unwieldy
and expensive for widespread use.
Over the following decades,
engineers further developed
this technology
and experimented with alternative
kinds of touchscreens.
Soon, resistive touchscreens
dominated the market.
But then, in 2007, Apple released
the first iPhone.
It was a breakthrough, yet it functioned
using the same principle
as the first touchscreen: capacitance.
Nowadays, capacitive and resistive
touchscreens
are two of the most common types.
Both use an external input
to complete their electric circuits.
In conductive materials,
electrons flow around atoms,
forming an electric current.
In contrast to insulators,
the electrons in conductors are
weakly bound and flow easily.
A resistive touchscreen has two layers.
The top is a clear, flexible material—
usually plastic—
while the bottom is something rigid,
like glass.
These layers are coated
with a conductive substance
and separated by a thin gap.
When something pushes
hard enough, the layers connect,
completing the electric circuit.
This causes a change in voltage
that the machine’s software reacts to.
Resistive touchscreens can
be a little unresponsive,
but they're generally cheap and durable,
so they're favored for industrial
or mass use.
A vast majority of the touchscreens
produced in 2007 were resistive.
But in the years following
the iPhone’s release,
most became capacitive.
Individual models vary,
but smartphone touchscreens today
typically consist of a protective,
insulating glass exterior
and an LCD screen at the bottom
that produces the images you see.
Between the glass exterior
and the LCD screen are several sheets.
One is lined with rows of a transparent,
conductive material
that carry an alternating
electric current.
A thin insulating layer separates these
conductive lines from others
that are arranged as columns.
One on top of the other,
the lines form a grid.
The points where they intersect
are called nodes.
The phone's battery draws electrons
along the first layer of lines,
and some electrons accumulate
at every node,
creating a small electric field.
These screens are called
capacitive touchscreens
because the nodes act like capacitors
by storing charge.
They’re generally easier to use
than resistive touchscreens
because they interact directly with your
finger without the application of force.
Your body is a great conductor and is
constantly transmitting electric currents.
Why? Because about 60% of you is water.
Now, while chemically pure water
is an insulator,
most water is impure.
The water inside you is loaded with ions—
atoms or molecules that have
a net electrical charge.
So when you click on an app, your finger
functions like a third electrical line.
It interacts with the existing
electric field,
which induces a weak electric current
that travels through your finger
and eventually back into the phone.
This changes the amount of charge
at the affected nodes.
And voltage measurements along
the second layer of lines
tell the phone’s microprocessor which
part of the screen is being touched.
However, if you try using a smartphone
while your hands are wet or gloved,
you'll probably have some trouble.
Both interrupt the electrical connection
between your finger and phone.
If water is splashed across the screen,
it might trigger many underlying nodes,
and the phone could act like you’ve
touched it in multiple places at once.
On the other hand, gloves are insulators,
so the charge has nowhere to go.
Meanwhile, objects that conduct
electricity about as well as your finger—
like banana peels
and certain processed meats—
can all activate the screen—
knowledge that can come in clutch,
when you’re in a pickle.
