Negative Resistance circuits anyone?

[Robert Baer]

"N" type:
Version 4
SHEET 1 880 680
WIRE 96 64 48 64
WIRE 208 64 96 64
WIRE 304 64 208 64
WIRE 96 96 96 64
WIRE 208 112 208 64
WIRE 304 128 304 64
WIRE -32 144 -32 64
WIRE -32 144 -96 144
WIRE 32 144 -32 144
WIRE 96 208 96 192
WIRE 96 208 -96 208
WIRE 208 208 208 192
WIRE 208 208 96 208
WIRE 304 208 304 192
WIRE 304 208 208 208
FLAG -96 144 In
IOPIN -96 144 In
FLAG -96 208 In
IOPIN -96 208 In
SYMBOL diode 288 128 R0
SYMATTR InstName D1
SYMATTR Value 1N4007
SYMBOL voltage 208 96 R0
WINDOW 3 -47 135 Left 2
SYMATTR Value Rm=16.3K
SYMATTR InstName Vm
SYMBOL npn 32 96 R0
SYMATTR InstName Q1
SYMATTR Value TIP50
SYMBOL res -48 80 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 3.58K
TEXT -112 176 Left 2 ;Vin
TEXT -88 272 Left 2 ;2-terminal negative resistance\n used for meter
protection.\n (plot Vm VS Vin)
..ENDS

Puzzling plot: SCR anybody?
Version 4
SHEET 1 880 680
WIRE 96 64 48 64
WIRE 256 64 96 64
WIRE -32 96 -32 64
WIRE 16 96 -32 96
WIRE 96 96 96 64
WIRE 256 96 256 64
WIRE -80 144 -96 144
WIRE 16 144 16 96
WIRE 16 144 0 144
WIRE 32 144 16 144
WIRE -96 224 -96 144
WIRE 96 272 96 192
WIRE 96 272 -32 272
WIRE -96 352 -96 320
FLAG -96 352 0
FLAG 256 176 0
SYMBOL npn 32 96 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res -48 80 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 58 -18 VBottom 2
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL pnp -32 320 R180
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL res -96 160 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R2
SYMATTR Value 0.1
SYMBOL voltage 256 80 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR InstName V1
SYMATTR Value 0
SYMATTR SpiceLine Rser=1E-6
TEXT 60 296 Left 2 !.dc V1 0V 100V 1mV
TEXT 88 -16 Left 4 !.PLOT V(V1), I(V1)
..ENDS

How can one emulate a "Z" type negative resistance?

Thanks.