A tiltmeter is an instrument designed to measure very small changes from the horizontal level, either on the ground or in structures. It uses a small container filled with a conducting fluid and a "bubble" to measure a change in slope. Electrodes placed in the fluid and into the bubble determine the bubble's position--as the bubble moves, voltage output from the electrode changes in a way that correlates to the amount of tilt that caused the bubble to move. Tiltmeters are used extensively for monitoring volcanoes, the response of dams to filling, the small movements of potential landslides, the orientation and volume of hydraulic fractures, and the response of structures to various influences. They measure the amount of tilt in microradians, which is the angle turned by raising one end of a beam one kilometer long the width of a dime. Originally designed as part of the guidance and control system for military missiles, a variety of tiltmeters are now available for volcano monitoring, each with different resolutions and ranges. For example, we use tiltmeters with ranges of between 100 and 10,000 microradians depending on the volcano and expected degree of tilt.