The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit
in Amsterdam.
It consists of a bulb containing mercury attached to a glass tube of narrow diameter; the volume
of mercury in the tube is much less than the volume in the bulb.
The volume of mercury changes slightly with temperature; the small change in volume drives
the narrow mercury column a relatively long way up the tube.
The space above the mercury may be filled with nitrogen or it may be at less than atmospheric
pressure, a partial vacuum.
In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium
with a temperature standard such as an ice/water mixture, and then with another standard such
as water/vapour, and the tube is divided into regular intervals between the fixed points.
In principle, thermometers made of different material might be expected to give different
intermediate readings due to different expansion properties; in practice the substances used
are chosen to have reasonably linear expansion characteristics as a function of true thermodynamic
temperature, and so give similar results.
History
The thermometer was used by the originators of the Fahrenheit and Celsius scales.
Anders Celsius, a Swedish scientist, devised the Celsius scale, which was described in
his publication the origin of the Celsius temperature scale in 1742.
Celsius used two fixed points in his scale: the temperature of melting ice and the temperature
of boiling water.
This wasn't a new idea, since Isaac Newton was already working on something similar.
The distinction of Celsius was to use the condition of melting and not that of freezing.
The experiments for reaching a good calibration of his thermometer lasted for 2 winters.
By performing the same experiment over and over again, he discovered that ice always
melted at the same calibration mark on the thermometer.
He found a similar fixed point in the calibration of boiling water to water vapour.
At the moment that he removed the thermometer from the vapour, the mercury level climbed
slightly.
This was related to the rapid cooling of the glass.
When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down",
i.e. he chose to set the boiling point of pure water at 0 °C and the freezing point
at 100 °C. One year later, Frenchman Jean-Pierre Christin proposed to invert the scale with
the freezing point at 0 °C and the boiling point at 100 °C. He named it Centigrade.
Finally, Celsius proposed a method of calibrating a thermometer:
Place the cylinder of the thermometer in melting ice made of pure water and mark the point
where the fluid in the thermometer stabilises.
This point is the freeze/thaw point of water.
In the same manner mark the point where the fluid stabilises when the thermometer is placed
in boiling water vapour.
Divide the length between the two marks into 100 equal parts.
These points are adequate for approximate calibration but both vary with atmospheric
pressure.
Nowadays, the triple point of water is used instead of the freezing point, 0.01 °C).
Before the discovery of the true thermodynamic temperature, the thermometer defined the temperature;
thermometers made with different materials would define different temperature scales.
In practice, several materials gave very similar temperatures to each other and, when discovered,
to the thermodynamic temperature.
Maximum thermometer
A special kind of mercury-in-glass thermometer, called a maximum thermometer, works by having
a constriction in the neck close to the bulb.
As the temperature rises, the mercury is pushed up through the constriction by the force of
expansion.
When the temperature falls, the column of mercury breaks at the constriction and cannot
return to the bulb, thus remaining stationary in the tube.
The observer can then read the maximum temperature over the set period of time.
To reset the thermometer it must be swung sharply.
This design is used in the traditional type of medical thermometer.
Maximum minimum thermometer A maximum minimum thermometer, also known
as Six's thermometer, is a thermometer which registers the maximum and minimum temperatures
reached over a period of time, typically 24 hours.
The original design contains mercury, but solely as a way to indicate the position of
a column of alcohol whose expansion indicates the temperature; it is not a thermometer operated
by the expansion of mercury; mercury-free versions are available.
Physical properties Mercury cannot be used below the temperature
at which it becomes solid, -38.83 °C. If the thermometer contains nitrogen, the gas
may flow down into the column when the mercury solidifies and be trapped there when the temperature
rises, making the thermometer unusable until returned to the factory for reconditioning.
To avoid this, some weather services require that all mercury-in-glass thermometers be
brought indoors when the temperature falls to -37 °C. To measure lower meteorological
temperatures, a thermometer containing a mercury-thallium alloy which does not solidify until the temperature
drops to -61.1 °C may be used.
Phase-out As of 2012, many mercury-in-glass thermometers
are used in meteorology; however, they are becoming increasingly rare for other uses,
as many countries banned them for medical use due to the toxicity of mercury.
Some manufacturers use galinstan, a liquid alloy of gallium, indium, and tin, as a replacement
for mercury.
The typical "fever thermometer" contains between 0.5 to 3 g of elemental mercury.
Swallowing this amount of mercury would, it is said, pose little danger but the inhaling
of the vapour could lead to health problems.
List of countries with regulations or recommendations on mercury thermometers
Argentina In February 2009, the Argentine Health Ministry
instructed by resolution 139/09 that all health centres and hospitals should buy mercury-free
thermometers and blood pressure meters and called on dentists, medical technicians, and
environmental health specialists to start eliminating this toxin.
As of 2011 mercury thermometers were still on sale to the public at pharmacies.
Austria There was a voluntary take-back action for
thermometers containing mercury based on the Federal Waste Management Plan 2006, and carried
out in close cooperation between the Austrian Chamber of Pharmacists, the Federal Ministry
of Environment, a private waste disposer, a producer of electronic thermometers and
a pharmaceutical distributor.
The disposal company supplied each pharmacy with a collection bin and covered the cost
of disposal.
The pharmaceutical distributor covered the logistical costs for the distribution of the
thermometers.
The pharmacies accepted a refund of only 0.50 Euro per thermometer for handling.
The supplier provided the thermometers at a reduced price.
The Federal Ministry supported each sold thermometer and advertised the project.
During the collection period, consumers could bring in a mercury thermometer and buy an
electronic thermometer for a subsidised price of 1 Euro.
Between October 2007 and January 2008, about 465,000 electronic thermometers were sold
and about one million mercury thermometers were collected.
Philippines By the Philippines Department of Health’s
Administrative Order 2008-0221, all mercury equipment from hospitals, including mercury-in-glass
thermometers, will be phased out in the Philippines by September 28, 2010.
Even before the order was released, 50 hospitals have already banned mercury from their establishments.
Among these fifty hospitals, the Philippine Heart Center was the first one to do so.
San Juan de Dios Hospital, Philippine Children’s Medical Center, San Lazaro Hospital, Ospital
ng Muntinlupa, Lung Center of the Philippines, the National Kidney and Transplant Institute,
Manila Adventist Medical Center and Las Piñas Hospital also made steps to ban the toxic
chemical.
The country was the first one to make a step to ban mercury from its health care system
in Southeast Asia.
United Kingdom Since European Union directive 2007EC came
into force on 3 April 2009, the UK Health Protection Agency reported that mercury thermometers
could no longer be sold to the general public.
Shops holding stocks of unsold thermometers had to withdraw them from sale; mercury thermometers
purchased before this date could be used without legal implications.
The purpose of these restrictions is to protect the environment and public health by decreasing
the amount of mercury waste released.
The HPA had, in 2007, released a guide to dealing with small spills of mercury.
United States of America In the United States, both the American Academy
of Pediatrics and the United States Environmental Protection Agency recommend that alternative
thermometers be used in the home.
See also Mercury probe, an electrical probing device
to sample for electrical characterization Mercury switch, an electrical circuit, on-off
switch using the element mercury Mercury swivel commutator, an electrical circuit,
current-reversing switch using the element mercury
Mercury vapour turbine, a rotary engine to produce electricity from mercury vapor
References
External links "guidelines for mercury spills removal".
US Environmental Protection Agency.