Arc knob

What it is

The Arc knob is a classic custom control for number columns. It draws a dial that sweeps most of a circle, with the value in the middle. The user drags the handle around the arc, or uses the keyboard, to raise or lower the value between a minimum and a maximum.

When to use it

Hardly ever for new work. Microsoft deprecated the Arc Knob in 2021 with several other older controls. A dial is also slower to use than typing and harder to make accessible. Use the Slider for a rough value, or the Whole number, decimal and float control for an exact one. The Linear gauge covers the read-only case.

Licensing and configuration

No extra licence. In the older form editor you select the column, open the Controls tab and choose Add Control. Microsoft's deprecation notice says these controls could no longer be added to forms from April 2021 and that existing instances would keep working until April 2022. The notice does not say what current environments do with existing instances, so test before you depend on it.

Pitfalls

• The control may not be offered when you add controls to a form in a current environment.
• Dragging around a circle is imprecise, especially on a touch screen, so users cannot reliably hit a specific value.
• Screen reader and keyboard support is weaker than in the standard number control.
• An existing form that uses it is a maintenance risk. Plan a replacement.

Try it in the demo

1. Open the Accelerator form at the deep link and scroll to the Custom controls section.
2. Drag the handle around the Arc knob. The number in the centre follows.
3. Click the knob, then press the arrow keys, Home or End to step through the range.
4. Press Ctrl+S to save the new value.