Four pairs of words are given here out of which the words in three pairs bear a certain common relationship. Choose the pair in which the words are differently related.

Ammeter: Current Hygrometers : Pressure Speedometer: Speed Seismograph : Earthquakes

Hygrometers : Pressure

Ammeter is used to measure the current, speed is measured by speedometer, seismographs are used to measure the intensity of earthquake. Whereas hygrometer measures humidity.