Made of a narrow band of spring steel, by careful manipulation, the tape can be guided through confined spaces such as wall cavities. The goal is to push toward an area where guide string has been dropped inside the confined space and to pull it through, so the guide string can then be used to pull through various types of wiring, such as phone wire, network cables or speaker wire. Fish tape is designed to pull through guide string only. Using it to directly pull the target wire can damage or warp the fish tape.
Fish tapes are usually stored coiled on a plastic reel. Because of this, they have a natural curvature and it is this curvature that allows them to be guided. By manipulating the reel, the end of the tape can be directed slightly. The tape is rigid enough that it can then be pushed in the direction in which it is pointing. In this way it can be easily guided through an empty wall cavity. Thermal insulation, firestops, pipes, HVAC ducts, electrical conduits, and other obstructions make use of a fish tape more challenging.
The "tape" can be made from many different materials including steel, fiberglass, and nylon. The tape usually has a special end ranging from a hook or loop to a specialized fastener device to allow the user to attach the tape to the guide string (or a very light cable) before pulling.
Occasionally, two fish tapes are used from opposite ends of the wall. Because they each have a hooked end, one fish tape is capable of catching the other, and the one tape can then be pulled back, carrying the second tape out with it. Electricians sometimes attach a battery and a doorbell or buzzer to two fish tapes so that when the ends of the tapes make contact with each other (within the wall), the bell rings.
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