What is Mercury Switch : Working & Its Applications

Mercury or Quicksilver is a heavy and silver-white metal and it is the only metal that is liquid at standard pressure & temperature. It conducts electricity but does not conduct heat. Mercury is extremely toxic, so it affects animals, plants, and humans once exposed to it. But because of its ability to conduct electricity & liquid form, it was used widely in positional and electrical switches known as mercury switches. In 1910, the different forms of mercury switches were invented & developed by Phelan. In 1920, he formed the Absolute Con-Tac-Tor Corporation to make & market contactor mercury switches. So, this article discusses an overview of a mercury switch and its working with applications.

What is Mercury Switch?

A switch that is used to open and close an electrical circuit through a small amount of mercury is known as a mercury switch. This switch is also called a mercury tilt switch. The mercury switch symbol is shown below.

Mercury Switch Symbol

Mercury Switch Construction

The construction of a mercury switch can be done by using a minimum of one or more sets of electrical contacts within a sealed glass envelope which includes a drop of mercury. This glass envelope may also have air, a vacuum, or inert gas.

Mercury Switch Working
Mercury Switch Working

How Does a Mercury Switch Work?

In the glass envelope, the gravity will pull the mercury drop constantly toward the lowest point. Once this switch is tilted in the right direction, the mercury drop will touch a set of contacts, so the electrical circuit will be connected throughout those contacts. Similarly, when the switch is tilted in the reverse direction then the mercury drop will move away from that set of contacts so that the electrical circuit will be not connected.

let us see how mercury switch works in a fire alarm:

These are the most common type of switch used in fire alarms. They work on pressure, so when a fire occurs, the mercury is displaced and the switch will send an electric signal to the alarm panel indicating that there is a problem. The mercury switches are activated by heat, so when a fire breaks out, it will activate the mercury switch and cause it to send an electrical charge to your alarm panel.

Mercury Switch Wiring Diagram

Mercury tilt switch module interfacing with Arduino Uno is shown below. This project is helpful in detecting the tilting or direction of an object & provides digital output depending on the direction. So it works like a normal ON/OFF switch and this switch can be controlled through tilting. This switch mainly includes a mercury drop & two contacts in a glass enclosure, where the mercury drop will short the two leads depending on the direction.

Mercury Switch Wiring Diagram
Mercury Switch Wiring Diagram

The required components in this interfacing mainly include Arduino Uno, LED, mercury switch module,220Ω Resistor, breadboard,  buzzer & Jumper Wires.

  • The mercury switch module includes three pins GND, VCC & DO.
  • The operating voltage of this switch ranges from 3.3 V to 5 V.
  • The maximum output current is 15mA.

The connections of the above circuit can be done like this.

  • The GND pin of the mercury switch is connected to the GND pin of Arduino Uno.
  • The VCC pin of the switch is connected to the 5V pin of Arduino Uno.
  • The output pin of a switch is connected to pin-2 of Arduino Uno.
  • The buzzer’s positive pin is connected to pin-3 of Arduino Uno.
  • The buzzer’s negative pin is connected to the GND.
  • The LED’s positive pin is connected to pin-13 of Arduino Uno
  • The LED’s negative pin is connected to the GND pin.

Once the mercury switch is not tilted, then mercury drop will be at the base & shorting the contacts. So this will activate the LED & the output will be LOW.

Similarly, when the switch is in a tilted position, the mercury drop will move away from the switch contacts. So this will deactivate the LED & the output will be HIGH.


void setup() {
pinMode(13, OUTPUT); // led control pin
pinMode(3, OUTPUT); // pin 3 is used to control the buzzer
pinMode(2, INPUT); // pin to read the mercury switch input
void loop() {
if (digitalRead(2) == 1)
digitalWrite(13, HIGH);
digitalWrite(3, HIGH);
digitalWrite(13, LOW);
digitalWrite(3, LOW);
In the above code, the Pin 13 & pin 3 can be defined as output pins of LED & Buzzer respectively and Pin2 is defined as the input to read the condition of the switch. Once the switch is inclined beyond a specific angle, and then its output goes HIGH, which is read within the infinite loop continuously. Once the pin-2 is read HIGH, then it is programmed to control both LED & Buzzer alternatively through some delay.

Mercury Added Switches

There are different Mercury-added switches available like float switches, tilt switches, pressure switches, temperature switches, etc. Here, the float switch is activated through a change within liquid levels. The tilt switch is activated through a change within the position of the switch. The pressure switch is activated through a change within pressure & temperature switch is activated through a change within the temperature.

Mercury switches are applicable in different consumer, industrial & commercial products, like space heaters, appliances, ovens, security systems, air handling units, pumps & leveling devices.

The following table gives the data about how much mercury is used in individual switches. For instance, a mercury tilt switch can have from 50 milligrams to almost 5 grams whereas a float switch may have mercury from 100 mg to 67 grams.

So, the average amount of mercury used in switches is shown in the below table. These switches are still manufactured and used in the USA.


Amount of Mercury used in Grams

Flame Sensor

Above 1

Float Switch

0.1 – 70

Tilt Switch

0.05 – 5


The advantages of mercury switches include the following.

  • Mercury switches provide extremely high surface tension.
  • The empty bulb of this switch is filled with inert gas, so it avoids the oxidation of electrodes. So mercury switch provides an extremely long operating life.
  • Less cost & simple to use.
  • It is observable because its color is silvery shining.
  • It is extremely strong because mercury does not fade away easily.
  • Contacts in the switch stay hygienic even if an inside arc is formed, the surfaces of contact are changed on each operation, thus the contacts don’t exhaust.
  • A small mercury drop has a low resistance, so these switches can carry useful amounts of current within a small size.
  • The contacts of this switch are enclosed, thus corrosion of the contacts is not possible.
  • Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don’t wear out.


The disadvantages of mercury switches include the following.

  • Mercury is dangerous to the environment and also to us, so using mercury is restricted across the world.
  • Material nature is toxic & it can perform simply on/off functions.
  • It cannot be used outside -38oC to +356 oC temperature range.
  • It is very costly.
  • Their slow operating rate will make them inappropriate for applications that need many operating cycles for each second.
  • Glass envelopes may be delicate, so they need flexible leads to avoid damage to the envelope.


The applications of mercury switches include the following.

  • These switches are applicable in industrial, consumer, and commercial products like space heaters, appliances, air handling units, ovens, leveling devices, pumps, security systems, etc.
  • These types of switches were widely used in the automotive industry for anti-lock braking systems, open door indicators, trunk lid lights, etc.
  • This switch is mainly used to detect small inclinations of a large angle.

Thus, this is all about an overview of a mercury switch. From the above information finally, we can conclude that these electrical switches open & closes a circuit once a mercury drop connects metal electrodes within a switch to close the circuit. Here is a question for you, what is the chemical formula of mercury?