An iron-free double focusing β spectrometer for precision work is described. Several conversion lines in the spectrum of Th B + C have been measured absolutely, using a method which eliminates the measurement of the magnetic field. By combining several such β spectroscopic data one can caculate the elementary constant combination h/ m0c. The result is in excellent agreement with results obtained by other means. This is a sensitive check on the accuracy of the individual Bϱ values of the different lines.