Electrochemical cell voltage is also referred to as electromotive force or emf. The electromotive force generated by motion is often referred to as "motional emf". The second role is to generate an electromotive force (EMF). This was the first observed instance of the development of electromotive force by electromagnetic induction. A changing magnetic field generates an electromotive force otherwise known as Faraday's law of induction. Hydrazine has a higher electromotive force of 1.56 V compared to 1.23 V for hydrogen. The direction of the electromotive force is given by Lenz's law. This property of electromotive force production is known as the Seebeck effect. Sometimes an analogy to water pressure is used to describe electromotive force. An active network contains one or more sources of electromotive force. Rowland intended this to indicate a direct analogy with electromotive force. Passive networks do not contain any sources of electromotive force.