No electrical job should be attempted without some way to see if the juice is actually flowing. A test light is the easiest way to do this since there is no dial to watch or readout to keep in your vision. If it lights up, you're good. No light, no juice.
While a test light can't tell you what's wrong with a circuit, it can tell you it's not working. If you're doing electrical diagnostic work, that can be half the battle. Using your tongue as a testing tool went out with baggy Guess jeans.
To use a test light, attach one end to the positive wire and the other end to the ground. The fact that you are just looking for a light means you can keep your eyes on your work and leave the tool in you periphery. You'll see it light up. You can also use it to test for either positive current or grounding by attaching one end to something you know is a good lead (like the battery) and using the other end to test a wire.
Check out a picture of a test light.
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