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- Helmholtz Coils -
Comments on aspects of design of the BH300/600/1300 family
It was looked for simplicity and robustness with the smaller weight, with the possibility of modifying the configuration of the coils to obtain a great versatility.
We have devised the system of 4 pillars fixed to a lower plate and another upper one. The coils are fixed with special fork brackets screwed to the pillars.
This system provides firmness to the set and great facility to place the device under study at the centre of the coils, being also relatively easy its assembly and its possible disassembly. The same support is used for the three versions of each size (one, two or three axes).
The inferior plate also makes of table to help to place the object under study, or the necessary instruments, in the coils (the plate has a threaded hole at its centre). Also it serves to locate the wiring channels and the connection terminal block for the pairs of coils.
The superior plate has a circular window to allow the passage of objects towards the centre of the coils. This plate can be removed if it were necessary, without harming too much the rigidity of the set (excepting for the one axis versions). Also it can be replaced by another plate done specially for some application in particular.
The plates are of foamed PVC, with a density similar to the one of the softwood, reason why they are easy to drill or to cut if it is necessary.
Each coil of a pair has its own terminal block with nuts to hold the terminals of connection cables. This makes possible to modify the connections in each coil as wanted and with enough easiness. An example would be the "anti-Helmholtz" set-up, to generate gradients instead of homogeneous fields (see a comment in below about this applied to the forms).
We developed specific techniques to curve "U" shaped profiles in circles very precise and to accommodate the winding in its channel. We use profiles of an aluminium alloy because they are not ferromagnetic, do not need exterior finishing coating and for being light but with a suitable mechanical rigidity.
Under these premises we obtain coils with a cross-sectional area the smaller as possible in practice, which results in a better access to its interior, among other benefits. Also this allows that the generated fields are practically the predicted ones for ideal Helmholtz coils (of infinitely small section).
These forms provide an optimal evacuation of the heat generated by the electrical current, which makes possible a relatively high current density in the windings.
We think that this is a feature unique to our Helmholtz coils (1)(Footnote), put into practise with the aim to obtain a maximum versatility of the sets.
The basic idea is to also use the aluminium forms like coils. It is a logical consequence of the fact that each one of these circular forms must have its ends electrically separated in order that does not behave like a short-circuited loop when the coil works with alternating current.
Therefore each form constitutes a coil of a single turn. We have made the connections between the two of each pair and between these and the general connection block.
This way we obtain an "extra" Helmholtz pair in each axis (denominated with the suffix "s" after the letter of the axis). The following simplified scheme shows the standard connection of pair X. For axes Y and Z it is applied the same.
We call Xs the pair formed by the forms of pair X. In the same way exist Ys and Zs. X-a and X-b are the two coils of pair X.
Applications of "In-Circuit Coils Forms":
At the moment we conceive two basic applications: like electrostatic screens and generating of small special fields.
Like electrostatic screens when, for example, the coils are used like sensors of magnetic flux in the measurement of the magnetization, or the magnetic moment, of some magnet or another piece. A proper connection of the forms could diminish the electrical noise remarkably when the signal is very low.
But more interesting they seem, nevertheless, like generating of field, for which we suggest the following two applications.
First it is the generation of a secondary homogeneous field to modulate the main field, by means of a source of current different from the one from the main pair.
Second it is the generation of small magnetic gradients to improve the homogeneity of the main field when it is tried, for example, to cancel the terrestrial magnetic field in a very high degree. This could be necessary due to gradients in the local field produced by the influence of neighbouring ferromagnetic masses, such as structures of the building, machinery, terrain features, etc.
Also it can be useful for some experiment in particular that needs a main field with a superposed gradient, static or variable.
In order to generate a gradient with a pair of coils these must be connected of the way that usually is called "anti-Helmholtz". In the following figure we show the same former Xs forms pair connected in this way.
The arrow indicates the modified connections, drawn up in heavier blue lines. Comparing this scheme with the previous aid to notice better the difference "Helmholtz / anti-Helmholtz".
We trust that the users will be able to find other utilities to this feature of our coils. We will be delighted to hear on it.
(1) - We have no found antecedent in relation to this detail of design. We would thank a lot for any reference about something similar done previously.
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Updated: 18 Jul 2007