2 Jolly's apparatus

• A spherical thin glass bulb (A) connected to a long capillary tube (B) (bent at two right angles), fixed on a vertical board that stands on a base with levelling screws.
• The other end of the capillary is joined by a rubber tube to a wide tube (C) which can be moved up and down along the board; there is a graduated scale.

Procedure

1. Read the atmospheric pressure from a barometer.
2. Put mercury in the bulb's side to take up part of the volume (about 1/7 of the bulb), so that the volume of the remaining air stays practically constant at all temperatures (mercury expands about 7 times as much as glass).
3. Immerse bulb (A) in a beaker of water; pour mercury into (C) until the mercury in the other branch rises to a fixed mark X.
4. Heat the water to boiling, wait for the temperature to become steady.
5. Move (C) until the mercury in the bulb-side arm returns to mark X (so the volume is again constant). The gas pressure P = P_a + h, where h is the height difference between the surfaces in the two arms.
6. Let the water cool to about 90 °C, adjust (C) so that the mercury returns to X, measure the temperature and height difference and calculate P again.
7. Repeat at several temperatures.
8. Plot pressure (vertical) against temperature in °C (horizontal). A straight line is obtained which, when extended backwards, cuts the temperature axis at −273 °C.
9. Calculate β_P = (P₁₀₀ − P₀)/(P₀ × 100). It is found to be 1/273 per °C for air and the same for all gases.