The immense computational power of today's processors cannot be fully utilized unless appropriate sensors and actuators are developed to link them with the outside world. A method is presented here to compare different sensors with each other and to evaluate their relative merits. On the basis of physical considerations, sensors are decomposed into two essential building blocks: a sensor shell and a sensing mean. A figure of merit is defined and 11 different types of sensors, having the same sensing shells, are analyzed and compared with each other.