Oscilloscope channels test Full wave bridge rectifier Working with logic gates JULIAN GARCIA OSORIO How to measure alternating current in lab Who needs resistors? Class9WatsonL

In this circuit the transistor act as a logic switch which is controlled by Switch Supply.

The Switch is set on (i.e. logic value of 1) when there is a positive base current. It gets off (i.e. logic value of 0) when there is a zero or a negative base current. 

Try fliping the polarities of the Switch Supply (Battery) and transistor will work as unplugged switch. 

You may want to see that how to use the transistors in this lab.