I am surprised nobody yelled "NO!!!". That is a good way to cause big surges in your wiring harness and damage electronics. I used to test alternators that way too, in my dumF kid days.
Before I would replace an alternator, I would verify it doesn't work in-place first. Otherwise, you may be blowing a lot of money and time, and not fix the real problem. Apply 12 V across the 2 field terminals, with nothing else connected to them. If one terminal is already grounded (most round-back alternators), ground it with a jumper to be sure. With 12 V across the field (engine running), you should see a lot of current output on the car's ammeter (or install a separate big in-line ammeter). If not, maybe just worn brushes, which you can fix in the car. To go further, verify that the field draws current.