The general theory of extinction of a-c. arcs is reviewed. The expulsion fuse is stated to depend upon the gas blast produced by rapid decomposition of fuse tube material under the heat of the arc. Comparison of interrupting capacities of a soapstone fuse tube and a fiber fuse tube supports this view. The effectiveness of the gas blast is stated to be due to the high degree of turbulence it introduces into the confined arc space. The theory of the action of such turbulence is discussed. Data are given as to the composition and volume of gas ejected from a fiber fuse tube. The voltage interrupting capacity vs. ampere characteristics as obtained experimentally, are given for a fiber tube, and a boric acid lined tube. The curves for boric acid lie much higher than for fiber. The characteristics were found to depen dvery materially on the number of half cycles of arcing. Reasons are given for the shape of the characteristics, and their dependence on length of arcing time. Data are given showing the effect of variation in size and shape of the tube section. The design problem is discussed of obtaining sufficiently large voltage interrupting capacity at smaller currents without the development of excessive pressures or unmanageably large volumes of flame at large currents. The use of flame suppressors for deionizing the issuing flame is mentioned.