Can you use this magnet to guide a metal plumbob inside a wall to a predrilled hole for fishing electrical wire from basement to attic?

If any metal in the wall it will stick to it. This is a very strong magnet and you may lose it in the wall if you use a cheap string...

Are you supposed to add thread locker?

If on fishing line you should not need to. I don't know why you would need to remove the ring from the magnet housing, but you could add some thread-lock.

ASIN	B074BTSR1L
Best Sellers Rank	#581 in Industrial & Scientific ( See Top 100 in Industrial & Scientific ) #1 in Lift Magnets
Brand	DIYMAG
Color	Single Hole
Date First Available	August 14, 2017
Is Discontinued By Manufacturer	No
Item Thickness	2.36 Inches
Item Weight	11.2 ounces
Item model number	CT60MM
Manufacturer	DIYMAG
Material	Neodymium
Number of Pieces	1
Product Dimensions	2.35"L x 2.35"W x 2.51"Th
Shape	Round
UPC	850077560200

DIYMAG Super Strong Neodymium Fishing Magnets, 500 lbs(227 KG) Pulling Force Rare Earth Magnet with Countersunk Hole Eyebolt for Retrieving in River and Magnetic Fishing,Diameter 2.36 inch(60 mm)

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