These filters are designed for use with the Stetzerizer Radio-Frequency Meter. The meter measures a type of radio frequency resulting when house wiring acts as an antenna for electrical current (also called "dirty electricity"). Anyone can use this simple system: Just plug in the digital Microsurge Meter; if it reads 30 or above, you need a filter in that outlet. (Click here for installation instructions.)The filter looks like an AC adapter and plugs directly into the outlet. (Then take another meter reading and you’ll see a lower number.) In most cases, about 20 filters are required for an average-size house. These filters work best for freestanding homes and offices. (Apartments, condos and other attached units are difficult to treat because of frequencies from other units will still be entering your space.) Please note that because the effects of these filters are cumulative (readings at other outlets will decrease after you plug in one filter), using the meter by itself, without any filters with which to help test it, will not give you the most accurate readings. We recommend starting with at least five filters when you buy a meter to test your home.