A new calorimetric method for a.c. loss measurements in superconductors has been developed over the last few years, based on a null detection heating method. The measurements have shown general agreement with theoretical predictions and with values obtained by other methods. More recently, a second version of the set-up has been built to solve some initial problems: mainly, the insufficient thermalization of the tape extremities and thermocouple reference junctions. These have been corrected by the use of copper centred feed-throughs. It was then found that the new set-up yields correct results but with the use of an exchange gas. The method has been validated through a comparison of the a.c. losses measured by the null calorimetric method for a copper tape with those deduced from standard electrical measurements. The new version of the null calorimetric method has been used to measure a.c. losses of silver and silver alloy sheathed Bi-2223 tapes. Dependencies on current and on frequency of the a.c. losses are correlated with the V-I curves, for a "poor condition" tape sample and a "good condition" tape sample, characterized by different n-values according to a fit V = k•I n .