pen-like run-along-the-ac-line "locate-the-break" tester?

pen-like run-along-the-ac-line "locate-the-break" tester?
I once saw an electrician find a break in an ac-cable (110 volts) by running some pen-like instrument along the outside of the cable, down its length. It maybe beeped at the break, or a light turned off when the voltage dissappeared, I forget.
Anyway, I'd now like to get one of these nifty things.
(1) What are they called?
(2) What brands?
(3) Prices? Price vs quality?
(I'm no professional; I'd just use the thing around home, only once in a while, but it's sure save a lot of time, sweat, and tears -- and blood too, maybe!)
So, suggestions?
Thanks!
David
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David Combs wrote:

http://www.buytester.com/multimeters/1330.asp
$17.00
The Classic Greenlee Volt Tick -- an industry best-seller. Over 1 million sold. And no wonder. Pros and do-it-yourselfers alike have found the Volt Tick an invaluable safety tool. Now, this new version also features an on/off switch for better-than-ever performance and value.
* Convenient pen size, fits in pocket or tool pouch.
* Bright LED and audible alarm if voltage is present.
* Use to detect the presence of voltage at outlets, lighting fixtures, circuit breakers, wires and cables, or find a break in a wire.
* Accessories included: (2)1.5 volt AAA batteries
* Voltage detection: 50-1000VAC
* Frequency range: 50-500Hz
* Overvoltage protection: IEC 61010-1 Category III,1000V
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This is Turtle.
They have them at Lowe's Stores in a few different models. I like the G & B Instruments Company -- Mod. # GVD-504A at about $11.99 and they have the fatter model / suppose to be better / for $12.99. I do hvac work and keep one in my pocket to keep from getting electricuted. Everytime you go to touch or cut a wire. you check to see if your going to get fired up or not. Good little tool.
TURTLE
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