Lilienfeld 1925 Form Two Metal Terminals
# Form Two Metal Terminals: The Gap Between Them Is the Real Design Variable
Every step so far has concerned the control electrode — positioning it, insulating it, fixing it flush. Forming the two current terminals introduces a different kind of decision entirely: where exactly to stop one copper coating and start the other. That gap is not incidental spacing left over after depositing two separate patches of metal — it is the one dimension that will directly set the resistance of the compound film Steps 7 through 9 have not even formed yet, because whatever conducting path eventually bridges the terminals has to span exactly this distance, and nothing shorter or longer.
## 1. The Gap Deposited Here Becomes the Resistance Measured Later
Once the copper-sulfur film forms across the terminal gap in Step 8 and Step 9, its resistance between the two terminals follows the same simple geometric relationship any uniform conductor does — resistivity times length over cross-sectional area — except here, the length is a quantity this step alone determines:
where $L_{\text{gap}}$ is the separation between the two terminal coatings set here, and $w$ and $t_{\text{film}}$ are the film's width and thickness, both still several steps away from existing. A gap made too wide produces a film resistance so high that Step 11's main bias struggles to drive meaningful current through it at all; a gap made too narrow leaves little room for Step 3's foil edge, already fixed in position, to sit meaningfully between the two terminals without crowding one of them. This step's own choice of $L_{\text{gap}}$ is therefore not a detail to be refined later — it is the first number in the entire sequence that directly becomes a measurable electrical property of the finished device.
## 2. Real Diagram: One Gap, Soon to Be One Resistance
## 3. The Coating Also Has to Stay Attached Through Everything Still to Come
Beyond setting a resistance the circuit will inherit, each terminal coating has to physically survive the sulfurization Step 8 performs — a chemical reaction run at elevated temperature directly on top of the coating, which stresses the bond between the deposited metal and the glass underneath it through exactly the same kind of thermal-expansion mismatch this series has seen cause trouble in an entirely different device. Treating delamination as its own fracture problem, the coating stays attached as long as the energy available to drive a crack along the metal-glass interface stays below the interface's own work of adhesion:
where $\varepsilon_{\text{thermal}}$ is the thermal strain imposed by the mismatch in expansion coefficients between the copper coating and the glass support during Step 8's heating, and $t_{\text{coating}}$ is the coating's own thickness. This is the same underlying logic as Step 2's bulk fracture criterion, now applied to an interface rather than the glass itself — a coating too thick, or deposited with too little adhesion to the glass beneath it, can delaminate during sulfurization before the device Step 5 is building toward ever gets the chance to conduct anything.
## Real Diagram: Resistance Climbs Directly With the Chosen Gap
## Form Two Metal Terminals's Place in the Process Lineage
Form Two Metal Terminals follows Step 4, Reassemble the Support, which fixed the flush foil position this step's coatings now have to be deposited around without crowding; it precedes Step 6, Preserve Metal Separation, which exists specifically to keep these two freshly formed coatings from ever touching the foil directly. It is the fifth step of this concept's construction sequence and the first point where a purely geometric choice — how far apart to place two patches of copper — becomes, by the time Steps 7 through 9 finish, an actual number on a data sheet: the resistance of the device's main conducting path.