360 STATE BOARD OF AGKICULTURE. 



a special study of the conduction of frictional electricity in regard to lightning 

 rods. It has long been established by Coulomb and others that the electricity 

 of a charged, conductor exits in a thin stratum at the surface, and this is a nec- 

 essary consequence of the repulsion of electricity for itself, every particle being 

 repelled from every other as far as possible. From this it was hastily assumed 

 that electricity in motion also moves at the surface ; but this was an inference 

 without physical proof until I commenced the investigation. I found from a 

 series of experiments that frictional electricity, that is, electricity of repulsive 

 energy, such as that from the clouds, does pass at the surface, but that galvanic 

 electricity, the kind to which Faraday, Danieils, De La Rive and others refer, 

 passes through the whole capacity of the conductor. This latter fact, however, 

 was previously established by others. I further found, that whenever a, charge 

 of electricity was thrown upon a rod explosively, however well connected the 

 rod was with the earth, it gave off sparks in the course of its length sufficient 

 to fire an electrical pistol and light tiocculent substances. I also found that in 

 sending a powerful discharge from a battery of nine (9) jars through a wide 

 plate that no electricity passed along the middle of the plate, but that it was 

 accumulated in its passage at the edges. 



From all my study of this subject I do not hesitate to say that the plan I have 

 given of lightning rods is the true one, and that a tube of a sufficient degree 

 of thickness serves to conduct the electricity as well as a solid, mass, provided 

 the thickness is sufficient to give free conduction. A very heavy charge sent 

 through a wire frequently deflagrates it, but no discharge from the clouds, or 

 which I have any knowledge, has ever sufficed to deflagrate a gas-pipe of an 

 inch in diameter. 



The plan of increasing the surface of a rod by converting the metal into a 

 ribbon is objectionable. It tends to increase the power of the lateral discharge, 

 and gives no increase of conducting power. 



Another fallacy is much insisted on, viz. : the better conduction of copper 

 than iron. It is true that copper is a better conductor of galvanic electricity, 

 which pervades the whole mass, but in regard to frictional electricity the differ- 

 ence in conducting capacity is too small to be of any importance. Iron is suffi- 

 ciently good in regard to conduction, and withstands deflagration better than 

 copper ; besides this, it is much cheaper. 



Yours truly, 



JOSEPH HENRY. 



Whenever I read these letters of Prof. Henry I am struck with the great 

 kindness of a man so eminent in science, who, amid so many and such weighty 

 cares and responsibilities, will yet take the time to write such letters to one so 

 little known as myself. They afford a striking illustration of the ennobling 

 influence of science upon her votaries. 



I am painfully conscious of the rashness of one in my position attempting to 

 refute or even question a scientific proposition advanced by Prof. Henry, but 

 especially on a question to which he has devoted so much time in original 

 research and experimental investigation. But here is a question in science 

 regarding the path of conduction of electricity of high tension through a metal- 

 lic rod. On one side we have the statement of Prof. Henry claiming that he is 

 the only man who has given special attention to the path of conduction of elec- 

 tricity of high tension, who states that conduction of this form of electricity is 

 at the surface of a conductor; ou the other side we have the statement of men 

 distinguished in science who affirm that this, like other forms of electricity, is 



