238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



instantaneous flow of current similar to that between plates of the 

 better known metals. Within a few seconds the current flow will 

 drop to a very small comparative value, providing the battery volt- 

 age is not too high. The order of the current flow will become that 

 of 1 milliampere and less with impressed direct current voltage up to 

 75 volts, when sulphuric acid of the strength ordinarily used for stor- 

 age batteries is the electrolyte. This drop in current flow will be 

 accompanied by the formation of a film on the tantalum anode, pre- 

 sumably of tantalum oxide. This film often shows beautiful irides- 

 cent colors. 



If a tantalum plate and a lead plate are placed in an electrolyte and 

 a source of alternating current of the usual commercial frequency is 

 connected to the tantalum and lead plates, the current flow in one 

 direction will be almost entirely shut off and a pulsating direct cur- 

 rent will be obtained. In such a set-up this flow of current is ac- 

 companied by electrolytic action with evolution of hydrogen gas at 

 the tantalum and oxygen at the lead. The action of the tantalum 

 is, therefore, such that electrons are permitted to flow from the tan- 

 talum to release hydrogen ions but are prevented from passing from 

 oxygen ions into the tantalum. 



The current derived from this apparatus may be utilized for charg- 

 ing storage batteries, for the electro-deposition of metals, and vari- 

 ous other electrochemical actions requiring a direct current. 



It is possible, by using two tantalum electrodes in a single cell, 

 so to rectify the current that both half waves of alternating current 

 pass in the same direction. This current may be smoothed out by 

 a suitable series of inductances and capacities to give what is prac- 

 tically a constant direct current. 



The efficiency of tantalum as a valve with respect to leakage of the 

 current varies with the impressed voltage, with the electrolyte, cur- 

 rent density, etc. Due to the fact that tantalum is very inert toward 

 the chemical action of solutions, there is a wide choice of electrolytes 

 and the life of the tantalum appears practically unlimited. 



For a charging set-up with a 6 to 8 volt battery the energy effi- 

 ciency is approximately 33if, per cent, which compares favorably 

 with rectifiers of the hot and cold electrode type and the mechanically 

 vibrating rectifiers. 



The tantalum battery charging rectifier is noiseless in operation, 

 has no moving parts, and requires attention in only one matter, which 

 it has in common with the storage battery itself — that of distilled 

 water being added to replace evaporated and decomposed water of 

 the electrolyte. 



In addition to functioning directly as a rectifier for obtaining con- 

 tinuous current, apparatus built along similar principles may be 



