lO.!.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



157 



ElectricQLl Ore Finding. 



Prospecting by TelepKorve. 



In the early part of June, Professor Silvanus Thompson, 

 F.R.S., delivered a kind of informal lecture on the l>aft- 

 Williams method of locating metalliferous deposits by 

 means of electricity : and the system, xyhich has been 

 the subject of investis;ation for some time, may now be 

 regarded as having passed from the uncertainty of ex- 

 periment into the sphere of practical usefulness. Its 

 usefulness has some limitations, some of which Professor 



The Apparatus. 



Photo by Burrows. 



Silvanus Thompson indicated. It depends for its success 

 on the difference in electrical conductivity displayed be- 

 tween the lode of metal which it is desired to locate and the 

 soil in which the lode is found. Therefore, although the 

 system has been undeniably successful in locating veins 

 of galena and of zinc blende, it does not follow that 

 its success would be equally marked in locating other 

 metals existing in other matrices ; and it is by no means 

 certain that the results could distinguish between a small 

 thickness of rich ore and a number of stringers containing 

 an equal amount of ore so distributed as to be com- 

 mercially worthless. Still, the system is capable of 

 showing great development ; it is at present by no means 

 a mere scientific curiosity ; and even if it were, it is well 

 worth attention. 



We may best begin its description by an illustration. 

 If a flow of electricity were to take place between two 

 points at the top and bottom of this page, the electric 

 flow would not take place in a single straight line, but 



would arrange itself in a number of lines with a greater or 

 less resemlilance to the lines of force between the poles of 

 a magnet. ]5ut if on the page a bar of metal were laid, 

 then the position of thes(> lines would be disturbed— as a 

 log in a pond would disturb the concentric ripples that a 

 stone thrown into the pond's middle would otherwise 

 produce. Similarly, if w'e cause a current to How 

 between two points on the earth's surface, a held of force 

 in the earth's crust is formed ; and, as Sir William 

 Preece showed some twenty years ago, the lines of flow 

 of the field can be studied with a telephone'circuit con- 

 nected to earth by portable electrodes. In Messrs. 

 Williams and Daft's apparatus, the two transmitting 

 electrodes (between which the current is to be sent) are 

 earthed usually about 100 yards apart. The circuit in 

 w'hich they are the two points is fed by an indiution coil 

 which can deliver a very heavy secondary (Iis(-lKirge into 



J'hul.i by Hill 



istening to the Telephonic Communicators. 



a glass condenser. Two spark gaps — in series and in 

 parallel — are inserted in the circuit. The breaks are of 

 two types ; one of the pendulum type, and one which is 

 designed to give a " make" of any desired length and a 

 break of unusual abruptness. The receiving (or tele- 

 phone) circuit, which is to explore the lines of force 

 created, consists of two telephone receivers, each of 500 

 to yoo ohms resistance, connecting to the exploring elec- 

 trodes through a series of parallel switch. 



While the current passing between the transmitting 

 electrodes is suffering its "make and break," the tele- 

 phones attached to the other or receiving electrodes, which 

 are immersed in the soil about seventy feet apart, enable 

 the investigator to " hear " the current as it passes. It 

 sounds in the telephone receiver like the tap of a wood- 

 pecker, and it can be heard even when the two telephone 

 electrodes are immersed in the earth several miles away 

 from the immersed transmitting electrodes. In practice, 

 it is not usual to explore at distances more than half a 



