How to Build a Hydrophone
Page 16

Place the lid on the film canister; try not to get any air bubbles. Having an air bubble may cause the film canister to compress and change shape due to the pressure in deep waters. This should be done over a sink so excess mineral oil can spill over. Wipe film canister down with soap and water to remove excess mineral oil. Using the black tape, tape the lid onto the film canister.