  We developed a new type of AC microcalorimeter based on a modulated-bath
technique for measuring the specific heat of small microgram samples in the
temperature range from 30-300 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 Peltier effect, so that the
oscillation is symmetrical about the temperature of the main bath. This avoids
the problem of DC offsets which plagues 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 Bi2.12Sr1.71Ca1.22Cu1.95Oy has been determined.
