All inductive devices, operating in d.c. circuitry, which are switched on and off (whether by a contact or by electronic circuitry) should have a diode connected across their coils to catch the inductive fly back.
http://www.4qdtec.com/catch.html
In this activity, you will investigate the input characteristics and the current transfer characteristics of a transistor. You will learn to test a transistor using a multi-meter.
http://www.radian.com.hk/alphyprac/showOneExpt.asp?exptcode=E19
Here is a handy zener diode tester which tests zener diodes with breakdown voltages extending up to 120 volts. The main advantage of this circuit is that it works with a voltage as low as 6V DC and consumes less than 8 mA current.
http://www.electronic-circuits-diagrams.com/testimages/testckt4.shtml
The polarity markings on LEDs (Light Emitting Diodes) seem to be a common cause of confusion. The polarity may be indicated by a flat section on one side of the body or by one lead being shorter than the other, but different manufacturers are not consistent as to which lead they are marking.
http://www.web-ee.com/Electronic-Projects/projects/led_test/index.htm
It can be used to determine device type (NPN or PNP) and the base
http://www.imagineeringezine.com/PDF-FILES/xistrtst.pdf
http://www.web-ee.com/Electronic-Projects/projects/led_test/index.shtml
http://www.tpub.com/neets/book21/87d.htm
provides the ability to test gain at various collector currents and breaddown voltage
http://www.sound.au.com/project31.htm#top
http://www.lh-electric.4t.com/bin/gonogo.gif
http://home.maine.rr.com/randylinscott/nov97.htm