A midge-repellent machine is a device that uses ultrasonic or electromagnetic technology to repel midges. These machines work by emitting high-frequency sound waves or electromagnetic fields that are designed to be unpleasant to midges, but not harmful to humans or other animals. The theory behind these devices is that the sound waves or electromagnetic fields will cause the midges to avoid the area where the machine is located. However, the effectiveness of these devices has not been extensively tested and there is some debate about whether they are truly effective at repelling midges.