Physics Teachers Forum

Jeff,
We can get Hertz's original papers from the University if that helps. I have one of Hertz's original papers, and can get the others. The paper I have is his seminal paper on proving the existence of EM waves by showing that a standing electromagnetic wave can be formed and he measures its wavelength. My German is not too crash hot so I can only understand bits and pieces of it. This is just to let you know that the original papers can be obtained if you want them. I don't think we can place them to be downloaded due to copyright issues, but I'll have to check on that.

Cheers,
Joe

I am also interested in Hertz Experiments. I was wondering if it is possible to make apparatus to replicate the experiment of simply producing a secondary spark in the receiver due to em radiation. Is there a good website that explains his measurement of the wavelength of radio waves? Thanks

Kerry,
there is an excellent article from the peer reviewed literature about Hertz in detail. It shows the different ways he tried to carry out his experiments and the difficulties he had. Unfortunately, it's an article you pay for, but the reference is as follows:

The first century of microwaves-1886 to 1986
Bryant, J.H.
Microwave Theory and Techniques, IEEE Transactions on
Volume 36, Issue 5, May 1988 Page(s):830 - 858

Google it to find it.

Our University subscribes to this journal so if you contact me (from the contact the author link on this website) I might be able to arrange something for you.

However, to answer your question - no, it would be very difficult for most school teachers to set up Hertz's original experiment (any one of them - they were all difficult). I don't suggest you attempt it unless you are familiar with setting up high power oscillating and transmitting circuits.

Hertz did not simply create a spark and then detect that spark. If only life was that simple. He needed to create an oscillating current (and therefore an oscillating spark) at a precise and predetermined frequency through an antenna, which must be a quarter of a wavelength long in order to transmit its full power. Yes he used an induction coil to initiate a spark in the gap of the antenna but the geometry and dimensions of his antenna determined the oscillating frequency. His detecting loop (there were a few designs - some square and some round) had to have a natural oscillation frequency identical to that of the transmitting antenna. By transmitting at one frequency, all the energy will go into that frequency and therefore there would be more energy arriving at the detecting loop at that frequency, which can be detected, and if he wasn't too far away, it would result in an oscillating spark in the gap of the tuned detecting loop.

He calculated the dimensions and geometry he needed in order to have this tuned transmitter and receiver set-up. Even then, he needed to create devices so that he can fine tune the frequency of his antenna. One design had metal spheres on the ends of the antenna rods. He achieved this fine tuning by moving the spheres along the rods. In another design he replaced the spheres with metal plates and tuned by varying the area of the plates.

Even after all that, he had a fair bit of trouble from reflections from the almost empty and large lecture hall in which he was doing his experiments. If you have something as small as a school class room, then you will have enormous trouble with reflections.

To give you an idea, Hertz's first transmitting antenna had a full length of 3 metres. His detecting loop was square and had a side length of 1 metre.

As he became better at it, he was able to increase the frequency and therefore shorten the antenna.