In

1929,

an

American

scientist

named

Warren

A.

Marrison

designed

a

quartz

clock.

Quartz

is

a

crystal

that

vibrates

when

electricity

flows

through

it.

This

keeps

the

hands

of

the

clock

moving

at

a

steady

rate.

In

1949,

an

atomic

clock

was

invented.

Atomic

clocks

are

so

accurate

that

since

the

1960s,

they

have

been

used

to

keep

the

official

time

for

the

world.

Scientists

continue

to

build

more

accurate

versions

of

the

atomic

clock.

The

latest

one

loses

only

about

one

second

in

20

million

years.

An

atomic

clock