  We report in this letter our analysis of a large sample of photospheric
vector magnetic field measurements. Our sample consists of 17200 vector
magnetograms obtained from January 1997 to August 2004 by Huairou Solar
Observing Station of the Chinese National Astronomical Observatory. Two
physical quantities, $\alpha$ and current helicity, are calculated and their
signs and amplitudes are studied in a search for solar cycle variations.
Different from other studies of the same type, we calculate these quantities
for weak ($100G<|B_z|<500G$) and strong ($|B_z|>1000G$) fields separately. For
weak fields, we find that the signs of both $\alpha$ and current helicity are
consistent with the established hemispheric rule during most years of the solar
cycle and their magnitudes show a rough tendency of decreasing with the
development of solar cycle. Analysis of strong fields gives an interesting
result: Both $\alpha$ and current helicity present a sign opposite to that of
weak fields. Implications of these observations on dynamo theory and helicity
production are also briefly discussed.
