Generally speaking, the drier the mix, the better the concrete, provided it is compacted thoroughly. Because vibrators can provide very efficient compaction their use is specified on all but the smallest jobs. They help to produce concrete that is stronger (and cheaper) than could be obtained otherwise. The most commonly used vibrator is the internal or spud type, generally considered on of the most satisfactory because it works directly in the concrete. When using the spud, these are six main points to watch that help toward good results: (1) move the vibrator frequently; (2) when the spud is pulled our after a period of vibration, make sure that the hole it made has closed otherwise a hole will be left in the finished concrete; (3) when using a spud vibrator with timber formwork do not let it touch the inside of the forms, any mark on the formwork will mean a mark on the finished concrete; (4) do not allow a vibrator to touch reinforcement when part of that reinforcement is embedded in concrete that has stiffened but which is not yet one day old; (5) when compacting a layer of concrete on top of another layer that is newly placed, push the vibrator at least four inches into the lower layer; (6) do not try to compact a layer of concrete more than two feet thick with a spud vibrator as air has to be forced to the surface and the spud is inefficient at a depth greater than this. External machines, which send vibrations through the formwork, are used when for any reason it is not possible to insert a spud, such as in very narrow sections or where these is very heavy reinforcement. The following points should be noted: (1) vibrators must be clamped firmly on the formwork; (2) they should not be more than three feet apart in any direction- but their effective range depends on their power and on the workability and amount of concrete being vibrated, so in some instances they may need to be quite close together; (3) concrete that is to be compacted by external vibrators of any kind should be fed in little by little. Placing it in fairly shallow layers gives a more uniform concrete to work on and prevents air being trapped as the lift is built up; (4) the top two feet of concrete in a wall or column should always be compacted with a spud vibrator if the reinforcement permits.