Added Bills of Materials

This is long overdue, but the following prototypes now each have a BOM (bill of materials) in OpenDocument Spreadsheet (.ods) format, including a breakdown of the raw material cost, in case you'd like to replicate them:

• Prototype 6
• Prototype 7
• Prototype 8
• Prototype 9
• Prototype 9.1
• Prototype 9.2

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• Newer post: 2022 Retrospective
• Older post: 3M Red Dot electrodes
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• Date: 2022-02-24