A sensor has been developed which allows measurement of the force exerted by the bow on the string (bow force) during violin performance. The bow force is deduced from measurement of the transversal force at the termination of the bow hair at the frog. The principle is illustrated with an experiment that demonstrates how the bending of the stick and variations in bow hair tension influence the measurements. The design of the sensor is described and performance characteristics are discussed. A thorough calibration procedure is described and tested. Finally, the use of the sensor is demonstrated through measurements in real playing situations.