<s>
A	O
mercury	B-Algorithm
relay	I-Algorithm
(	O
mercury	O
displacement	O
relay	O
,	O
mercury	O
contactor	B-Algorithm
)	O
is	O
a	O
relay	O
that	O
uses	O
mercury	O
as	O
the	O
switching	O
element	O
.	O
</s>
<s>
They	O
are	O
used	O
as	O
high-current	O
switches	O
or	O
contactors	B-Algorithm
,	O
where	O
contact	O
erosion	O
from	O
constant	O
cycling	O
would	O
be	O
a	O
problem	O
for	O
conventional	O
relay	O
contacts	O
.	O
</s>
<s>
Owing	O
to	O
environmental	O
considerations	O
about	O
the	O
toxicity	O
of	O
mercury	O
,	O
mercury	B-Algorithm
relays	I-Algorithm
are	O
mostly	O
obsolete	O
though	O
modern	O
encapsulated	O
units	O
still	O
have	O
applications	O
.	O
</s>
<s>
They	O
are	O
generally	O
being	O
replaced	O
by	O
solid	B-Algorithm
state	I-Algorithm
relays	I-Algorithm
.	O
</s>
<s>
Mercury	B-Algorithm
relays	I-Algorithm
consist	O
of	O
a	O
vertical	O
(	O
usually	O
glass	O
)	O
tube	O
containing	O
liquid	O
mercury	O
.	O
</s>
<s>
Around	O
the	O
top	O
part	O
of	O
the	O
tube	O
is	O
placed	O
the	O
coil	B-Algorithm
.	O
</s>
<s>
When	O
energised	O
,	O
this	O
coil	B-Algorithm
attracts	O
the	O
slug	O
,	O
lifting	O
it	O
upwards	O
and	O
out	O
of	O
the	O
mercury	O
pool	O
.	O
</s>
<s>
The	O
control	O
coil	B-Algorithm
is	O
mounted	O
below	O
the	O
rest	O
level	O
of	O
the	O
slug	O
,	O
and	O
when	O
energised	O
draws	O
down	O
the	O
slug	O
deeper	O
into	O
the	O
pool	O
,	O
thereby	O
displacing	O
additional	O
mercury	O
and	O
thus	O
raising	O
the	O
level	O
to	O
the	O
previously	O
uncovered	O
side	O
contact	O
and	O
closing	O
the	O
circuit	O
.	O
</s>
<s>
The	O
mercury	B-Algorithm
relay	I-Algorithm
thus	O
allows	O
for	O
switching	O
of	O
higher	O
currents	O
with	O
a	O
small	O
control	O
current	O
,	O
for	O
a	O
large	O
number	O
of	O
cycles	O
.	O
</s>
<s>
The	O
glass	O
tube	O
of	O
a	O
mercury	B-Algorithm
relay	I-Algorithm
must	O
be	O
mounted	O
near-vertically	O
.	O
</s>
<s>
Mercury	B-Algorithm
relays	I-Algorithm
have	O
also	O
been	O
produced	O
as	O
latching	O
or	O
impulse	O
relays	O
.	O
</s>
<s>
Owing	O
to	O
the	O
mass	O
of	O
mercury	O
moved	O
during	O
switching	O
,	O
compared	O
to	O
that	O
of	O
the	O
armature	O
and	O
spring	O
leaves	O
of	O
a	O
conventional	O
relay	O
,	O
the	O
mercury	B-Algorithm
relay	I-Algorithm
is	O
not	O
a	O
high-speed	O
device	O
.	O
</s>
<s>
Despite	O
this	O
,	O
the	O
mercury	B-Algorithm
relay	I-Algorithm
does	O
have	O
a	O
very	O
low	O
contact	O
bounce	O
time	O
,	O
in	O
the	O
sub-millisecond	O
range	O
.	O
</s>
<s>
A	O
relay	O
,	O
usually	O
a	O
reed	B-Algorithm
relay	I-Algorithm
,	O
has	O
its	O
contacts	O
coated	O
with	O
a	O
small	O
quantity	O
of	O
mercury	O
.	O
</s>
<s>
This	O
gives	O
the	O
low	O
bounce	O
advantage	O
of	O
mercury	O
,	O
although	O
the	O
current	O
capacity	O
is	O
still	O
limited	O
to	O
broadly	O
that	O
of	O
the	O
original	O
reed	B-Algorithm
relay	I-Algorithm
.	O
</s>
