We developed a type of ac microcalorimeter based on a modulated-bath technique for measuring the specific heat of small microgram samples in the temperature range from 30 to 300 K, and tested it in magnetic fields up to 14 T. The device is built from a modified commercial Peltier element. The temperature of its top plate can be modulated periodically by the Peltier effect, so that the oscillation is symmetrical about the temperature of the main bath. This avoids the problem of dc offsets which plague conventional ac calorimeters. The sample is attached to a thin thermocouple cross, acting as a weak thermal link to a platform. The absence of a heater reduces the background heat capacity ("addenda") to a minimum. As an illustrative example of the performance of our device, the specific heat in fields up to 14 T of a small single crystal of the high-temperature superconductor Bi_(2.12)Sr_(1.71)Ca_(1.22)Cu_(1.95)O_(y) is determined.
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