Admittedly, I am very new to these things so I might have a misunderstanding, but based on what I've read I was under the impression that one of the functions of a transistor is to allow a high-power device (like a DC motor) to be able to safely draw more current from a relatively low-power device (like the Arduino). Or am I confusing things?
A more proper statement might read:
A transistor allows a lower voltage or power signal like an arduino output pin to CONTROL the transistor such that it can turn on and off a higher power device like a motor, solenoid, relay, etc. Another advantage is that a transistor switching circuit can have it's collector circuit operationg at a higher voltage (say +12vdc) while being CONTROLLED for a 5vdc signal like an arduino output pin. So keep in mind that controlling something and powering something are two different things, but both are required, proper voltage at proper current capacity and a proper control signal. An Arduino is great at CONTROLLING things by using its output pins and input pins under program control via your sketch, but these are just control signals. An arduino is only useful at powering things if they work at the +5vdc the arduino works at and even then there is only about 500ma or so of current available for external devices using that +5vdc voltage. External power supplies are the normal method to power external devices and circuits requiring other then +5vdc and at less then 500ma.
Lefty