gyms have a free motion machine. If you don't know what a free motion is, or have never used one before you should try one out.
A free motion is like a condensed cable crossover.
This set of dual cable biceps curls is a great isolator of the biceps because your body is motionless with no way to cheat the common way people cheat on biceps curls with the lower back.
Keep the elbows up towards the ceiling and keep the wrists in neutral alignment and curl the cables towards your shoulders.
You can also pop back up using the resistance to help you and immediately transition into dual cable triceps curls to really pump up your arms.
The free motion also allows you to work each biceps independently.
If you are looking to build bigger muscles please read the simple facts about muscle building here:
Trainer Mike Behnken, MS, CSCS