OF ARTS AND SCIENCES. 405 



American Alsinece. II. New and hitherto imperfectly known 

 Plants collected in Mexico by C. G. Pringle in 1892 and 

 1893. III. Notes upon the Genus Galinsoga. 



On an Apparatus for the Measurement of Coefficients of 

 Self-induction and the Investigation of the Phenomena of 

 Alternating Currents. By Frank A. Laws. Presented by 

 Charles R. Cross. 



Eight hundred aud sixty-fourth Meeting. 



February 14, 1894. — Monthly Meeting. 



The Academy met at the Walker Building of the Massa- 

 chusetts Institute of Technology, Boston. 



The President in the chair. 



Elihu Thomson showed and described a number of phe- 

 nomena produced by electrical discharges of high potential 

 and of high frequency. 



Work in this field began with Henry, and a considerable amount 

 of experimentation in the same field has recently been carried on by 

 Tesla, and others. 



To obtain high frequency discharges, the current of an alternating 

 current dynamo of 1,000 volts was reduced to 100 volts by suitable 

 lighting transformers. This low pressure current was passed through 

 the primary of a coil resembling a Ruhmkorff" without break-piece, 

 and raised to an alternating current of from 20,000 to 30,000 volts 

 in the secondary, such current having still the original frequency of 

 120, that is 120 complete waves per second. This high pressure 

 current was led directly into a large condenser', the metallic foils of 

 which were insulated by a heavy mica sheet and contained in boxes 

 filled with oil. The condenser, charged to 20,000 or 30,000 volts, in 

 discharging jumped spaces or gaps between balls of brass through 

 which spaces a strong air jet was kept blowing (Figure 1). The 

 discharge circuit of the condenser includes these gaps, and may 

 also include various coils, or plain wires, or paths designed to be trav- 

 ersed by the condenser discharge. If the condenser discharge takes 

 place over a short path, of say otdy a few feet total length, the fre- 

 quency or rate of alternation of the current composing the discharge 

 may be several hundred thousand per second. This rate is so high 



