The original concept of interferential therapy was developed over 57 years ago by Austrian physician, Dr. Hans Nemec. He found that by crossing two slightly different medium frequency alternating currents within the tissue; a third frequency current of greater intensity is created in the deeper tissue, effectively delivering deep therapeutic benefits for pain control and speeding up the healing process while maximizing patient comfort. Features & Specifications IF 4000 Interferential, Analog Dual Channel Interferential Carrier Frequency: 4000Hz Fixed Difference Frequency: 1-150Hz Adjustable Four Frequency Shifts: Continuous, 1/1 Abrupt, 8/8 Abrupt, 10/10 Ramped Wave Form: Symmetrical Bi-Phasic Square Power Source: 9 Volt DC Adapter or *Alkaline 9 Volt Battery 1 Year Manufacturer Warranty Includes: Stimulator, Lead Wires, 1 Package of Self-Adhesive Electrodes, 9 Volt DC Adapter, *9 Volt Battery, Hard Plastic Carrying Case, Instruction Manual Private labeling available (carrying case only)