Why is the bulb in a thermoniter important?

Answer 1

The bulb in a thermometer increases the sensitivity.

When the temperature changes in the bulb, the Mercury or other fluid will go up or down the narrow tube. The reason for this is due to change in the volume caused by expansion of the mercury or fluid; as the temperature increases, the volume increases.

However, the change in volume may be very small, so by having a narrow tube, any movement caused by expansion will be more visually noticeable.

To illustrate this, imagine two tubes containing 10ml of fluid each at 20oC. One tube is 1cm in diameter, the other 0.1cm. By using the equation for the volume of a cylinder we can calculate the height of the fluid in the tubes, (remember that 1ml = 1cm3)

In 1cm diameter tube:
Volume of fluid = pi x radius2 x height of fluid in tube
10cm3 = pi x (0.5cm)2 x height of fluid in tube
Height of fluid in tube = 10 / ( pi x 0.52 ) = 12.73cm

In 0.1cm diameter tube:
Volume of fluid = pi x radius2 x height of fluid in tube
10cm3 = pi x (0.05cm)2 x height of fluid in tube
Height of fluid in tube = 10 / ( pi x 0.052 ) = 1273cm

As can be seen, the height of fluid in the smaller tube is much greater as the same amount of fluid has to fit into a smaller space. Now imagine that for a 1oC change in temperature the fluid expands 1% in volume. So at 21oC the 10ml of fluid expands 1% to 10.1ml (10ml x 1.01 (1%) = 10.1ml). Now we can calculate how much movement this would create in the tubes

In the 1cm diameter tube:
Volume of fluid at 21oC = pi x radius2 x height of fluid in tube at 21oC
10.1cm3 = pi x (0.5cm)2 x height of fluid in tube at 21oC
Height of fluid in tube at 21oC = 10.1 / ( pi x 0.52 ) = 12.86cm

So the fluid in the 1cm diameter tube has moved:
Height at 20oC - Height at 21oC = Movement of fluid
12.86cm - 12.73cm = 0.13cm

Whilst in the 0.1cm diameter tube the fluid has moved:
Height at 20oC - Height at 21oC (calculation not shown) = Movement of fluid
1286cm - 1273cm = 13cm

As you can see, the movement in the smaller tube would be much more noticeable than in the larger tube and this would mean that even very small temperature changes could be visualised as the fluid moves a lot further. So that is why the central tube is so small, but why the bulb?

In the example just shown both tubes had the same volume of starting fluid, 10ml. But, as you saw, 10ml in a 0.1cm diameter tube requires a tube of over 12 metres, a very impractical size for a thermometer! One way to solve this is to reduce the amount of starting fluid to 0.1ml.

In the 0.1cm tube:
Volume of fluid = pi x radius2 x height of fluid in tube
0.1cm3 = pi x (0.05cm)2 x height of fluid in tube
Height of fluid in tube = 0.1 / ( pi x 0.052 ) = 12.73cm

Now we have a thermometer, which although large, could be used. But, look what happens when we increase the temperature to 21oC. The fluid expands to 0.101ml (=0.1ml + 1%) and the new height of fluid in the 0.1cm diameter tube is:

Volume of fluid at 21oC = pi x radius2 x height of fluid in tube at 21oC
0.101 cm3 = pi x (0.05cm)2 x height of fluid in tube at 21oC
Height of fluid in tube at 21oC = 0.101 / ( pi x 0.052 ) = 12.86cm

This gives a change in height from 20oC to 21oC of only 0.13cm. We have lost the sensitivity given by the smaller tube.

Therefore, to make a sensitive thermometer we need a large volume and a narrow tube. So how do we do this without making a thermometer too big to use? By adding the bulb. The bulb is a lot wider than the narrow tube of the thermometer and is therefore able to hold a larger amount of mercury or fluid. But if the entire tube was the same width as the bulb, any expansion would be difficult to see. So by combining the bulb, with its ability to hold a large volume of mercury or fluid, with the narrow tube, with its ability to cause large movement of fluid for a small amount of expansion, we create a sensitive thermometer.

So to give a short answer, the bulb increases sensitivity by increasing the volume of mercury or fluid inside the thermometer causing greater amplification of expansion, or movement, up the narrow tube. So more volume = greater movement = greater sensitivity.

The bulb in a thermometer increases sensitivity. It helps determine whether the fluid will go up or down. The bulb helps make a sensitive thermometer and it helps in holding a larger amount of mercury or fluids than the narrow tube.

Answer 2

The bulb in a thermometer contains a special liquid that expands or contracts with temperature changes, allowing the thermometer to measure and indicate temperature accurately. It serves as the sensing element that reacts to temperature variations by changing its volume, which is then displayed on the thermometer scale. Without the bulb, the thermometer would not be able to measure temperature effectively.