How To Activate a Conduit : Activate the Conduit on Minecraft PE
The following is also from the conduit page:
When activated, conduits give the “Conduit Power” effect to all players in contact with water or rain, within a spherical range of 32-96 blocks. This effect stops the breath meter from decreasing, gives underwater night vision and increases mining speed. Conduits can be activated in any biome and at any height or depth.
To activate, a conduit needs to be surrounded on all sides by water (source blocks, flowing water, and/or waterlogged blocks) and the conduit and water must be enclosed in an activation frame. The frame is built up of blocks in three 5×5 open squares centered on the conduit, one around each axis. Only prismarine, dark prismarine, prismarine bricks and sea lantern blocks in the frame contribute to activation. A minimum of 16 blocks are required, and produce an effect range of 32 blocks. Prismarine-type slabs (including double slabs) and stairs cannot be used to activate the conduit.
The effect range of the conduit is 16 blocks for every seven blocks in the frame, though the effect does not activate until the minimum of 16 blocks are included in the build. Thus, it extends to 48 at 21 blocks, 64 at 28 blocks, 80 at 35 blocks, and 96 with a complete frame of 42 blocks. A complete frame also carries the additional advantage of attacking hostile mobs within 8 blocks of the conduit by 4 every 2 seconds, if they are in contact with water or rain.
Other blocks used as part of the frame (including prismarine stairs, slabs, and double slabs) have no effect on activation, and can be omitted. Any blocks outside the frame but within the 5×5×5 cube that encompasses it likewise have no effect.