To draw the B-H curve of a specimen in the form of a transformer and to determine its hysteresis loss, the following procedure can be followed:

1. Preparation of the specimen: A specimen in the form of a small transformer should be prepared. The core material of the transformer can be made of laminated iron or any other suitable magnetic material.
   
2. AC excitation: Apply an AC voltage to the primary coil of the transformer. This will cause the core to magnetize and demagnetize periodically, leading to cyclic changes in the magnetic flux density (B) and magnetic field strength (H).
   
3. Measurement of B and H: Measure the magnetic flux density (B) and magnetic field strength (H) at various points in the cycle. This can be done by using a Gauss meter for measuring B and a Hall effect sensor for measuring H.
   
4. Plotting the B-H curve: Plot the B-H values on a graph with B on the vertical axis and H on the horizontal axis. Repeat the measurement and plotting process for different AC excitation frequencies and amplitudes.
   
5. Calculation of Hysteresis Loss: The hysteresis loss can be calculated using the area enclosed by the B-H curve. This area represents the energy lost in the magnetic material due to hysteresis. The formula for hysteresis loss (P) can be expressed as P = f * B * dB / dt, where f is the frequency of the AC excitation, B is the magnetic flux density, and dB/dt is the rate of change of B with time.
   
6. Repeat the experiment with different core materials and compare the hysteresis loss values to observe the effect of different core materials on hysteresis loss.
   

Note: It is important to maintain a constant temperature during the experiment to avoid any errors in the measurement.