We contribute a thin, transparent, and low-cost design for electric field sensing, allowing for 3D finger and hand tracking, as well as in-air gestures on mobile devices. Our approach requires no direct instrumentation of the hand or body, and is non-optical, allowing for a compact form-factor that is resilient to ambient illumination. Our simple driver electronics are based on an off-the-shelf chip that removes the need for building custom analog electronics. We describe the design of our transparent electrode array, and present a machine learning algorithm for mapping from signal measurements at the receivers to 3D positions. We demonstrate non-contact motion gestures, and precise 3D hand and finger localization. We conclude by discussing limitations and future work.