Page:Popular Science Monthly Volume 48.djvu/761

Rh than before, because the magnetic force is stronger the nearer we get to a. Finally, let b touch a. We have then really only one wire, since the wires touch and form one conductor. Of course, now the magnetic force can not send a current through our wire, as it did through b, in the opposite direction to the current from the battery; but it tries to do so, opposing the current from the battery. Consequently, the current that the battery gives is very weak for a short time, but only for a short time, because this opposing current lasts only while this magnetic force is growing. This phenomenon evidently holds for every wire through which we try to start a current.

The instruments used in the experiments between Cambridge and St. Louis could not work unless the current from the battery had reached its full strength, so that the time the experimenters found between the sending of the signal and its receipt was not the time it took for electricity to pass from Cambridge to St. Louis, but was the time it took for the current they used to grow to its full strength.

We know that the strength of the magnetic force around a wire depends upon the size and form of the figures into which the wire is bent, and the time it takes for a current through the wire to reach its full strength depends upon the strength of the magnetic force. Therefore we should expect that, by using different instruments on which wire is wound in different forms and sizes, we ought to find that it takes different times to send a signal from one place to another. This has been tried and found true. In fact, it was in this way that it was first proved that the velocity found between Cambridge and St. Louis was not really the velocity of electricity.

It would seem, then, that in our search for connecting links between electricity and light we had better turn our attention to what goes on in the space around a wire carrying a "current" rather than to confine ourselves to what takes place in the wire.

The Davenport Academy of Sciences is endeavoring to organize a systematic and thorough field work in archæology through the State of Iowa, with the expectation of ultimately publishing a final report on the subject. For that purpose it asks the co-operation of workers everywhere in the State in collecting the material necessary, hoping to accomplish the task in not less than five years. That the work may be intelligently done, it has sent out a "cirular of suggestion," giving details of instruction as to methods of proceeding in examining mounds, earthworks, shell heaps, village sites, rock shelters, aboriginal workshops, cliff carvings and paintings. A combined summary of what has already been done in this work has been prepared by Prof. Frederick Starr, and is sent out by the academy.