A heater containing viscous fluid, which is sheared to generate heat. The heater is provided with an electromagnetic solenoid that includes a case, a solenoid coil housed in the case, and a core extending through the case. The heater further has a heating chamber and a sub-oil chamber. Viscous fluid is moved between the heating chamber and the sub-oil chamber through a communication bore. The core extends toward the communication bore. A coil spring urges the core toward the communication bore. Excitation of the electromagnetic solenoid moves the core away from the communication bore. The core opens and closes the communication bore and pumps the viscous fluid into the communication bore when the solenoid is cycled on and off. As a result, the heat output is accelerated.