What makes an electrical device draws a current
Current will flow when the circuit is closed. If you have an open circuit it acts as infinite impedance, thus no current will flow. But as soon as you close the circuit (connect terminals of a voltage source) current will flow. 
Let's say you complete the circuit with copper wire (aka short circuit), this is essentially no impedance and the current will shoot to infinity. Now of course, in reality voltage sources have current limiters and will not allow the current to shoot to infinity. 
So open circuit equals infinity impedance (no current flow) and short circuit equals zero impedance (infinite current flow). 
As you decrease the impedance, the current drawn from the source increases and vice versa.
Good luck!